Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
I use a lot of fly fishing, entomology, and photography jargon on this site. The site incorporates these definitions
wherever they're used, but it might help some people to see them all listed in one place.
abdomen (abdomens, abdominal) - The rear and usually the longest (ten-segmented in mayflies) portion of an insect's body, to which the tails are attached.
acute - Characterized by a sharp angle, typically less than 90º, although sometimes used in a relative way to distinguish very sharp from somewhat less sharp angles.
adventive - Introduced but not fully established, used in reference to organisms living outside their native range.
aedeagus (aedeagal) - The reproductive (copulatory) organ of male arthropods
alkaline (alkalinity) - Having a pH higher than 7 (opposite of acidic). Moderately alkaline water is ideal for trout because it's better for the growth of phytoplankton, the usual base of the aquatic food chain, and that's good for the growth of everything higher up the chain, including trout.
alula - Membraneous lobes at the posterior angle of the wing base in some Diptera
anal cell (anal cell) - A cell posterior to an anal vein near the base of in an insect wing.
anal claw (anal claws) - Hook-like claws located on appendages of the last abdominal segment in some larvae, most commonly caddisfly larvae.
anal region (anal regions) - The most posterior portion of an insect's wings, containing the anal veins.
anastomosed (anastomosis, anastomoses) - Anastomosed wing veins branch out and rejoin, often creating the appearance of irregular subdivisions in the commonly expected pattern of veins.
anterior (anteriorly, anterad) - Toward the front of an organism's body. The phrase "anterior to" means "in front of."
anterolateral (anterolaterally, anterolad) - Located to the front and side.
anteromesal (anteromesally, anteromedial, anteromedially) - Located in the middle on the lateral axis, but toward the anterior end of a body or structure.
anteronotal - Located toward the anterior edge of the notum.
aperture (apertures) - The aperture is one of the main settings a camera or photographer determines before taking a picture. It is the diameter of the opening in the inside of the lens through which light can pass, and it varies from picture to picture.
apex (apices, apical, apically) - The apex is uppermost, outermost, or culminating point; the tip. Features located at the apex are described as apical.
apicolateral - Located to the side of a structure near its tip, or apex.
arculus - A small crossvein extending from the radius vein to the cubitus near the base of an insect wing.
asynchronous - The same generation hatching at different times.
atrophy (atrophied) - A body part which has shrunk or degenerated into a dysfunctional state is said to have atrophied. The mouth parts in adult mayflies are good examples.
attractor (attractors) - Flies not designed to imitate any particular insect, but to incorporate characteristics attractive to trout. When trout aren't feeding selectively, attractors often outperform careful imitations as searching patterns because they are easier to see and incorporate more strike-triggering characteristics. They include legends like the Adams, Bivisible, and Royal Wulff.
basal (basally) - close to the base; root or beginning
basisternum (basisternal, basisternite) - The primary, central plate in the anterior portion of an abdominal sternum.
behavioral drift (behavioral drifting) - The nymphs and larvae of many aquatic insects sometimes release their grip on the bottom and drift downstream for a while with synchronized timing. This phenomenon increases their vulnerability to trout just like emergence, but it is invisible to the angler above the surface. In many species it occurs daily, most often just after dusk or just before dawn.
bifid - Forked or otherwise divided into 2 distinct parts.
bilobed - Made of two distinct lobes.
bivoltine - Producing two generations per year.
brachypterous - Having short or or abbreviated wings.
brood (broods) - When a species produces more than one generation per year, each one is called a brood.
bulla (bullae) - An expanded point usually about half-way up the front edge of a mayfly wing, usually in the subcostal vein (first major vein not on the edge).
carapace (carapaces) - A hard, protective shell on the back of an animal. In mayflies, this refers to the enlarged section of the mesonotum which forms the "humps" of Baetisca nymphs.
carina (carinae) - A narrow keel-shaped anatomical ridge.
caudal - Toward the posterior tip of the body.
caudal cercus (caudal cerci) - A technical term for tail.
caudal filament (caudal filaments) - A technical term for tail. It may refer to any of the tails of a three-tailed insect, including the middle.
cephalad - Toward the head or anterior end of the body.
cercus (cerci) - The left and right "tails" of an insect are known as the cerci or caudal cerci. The middle tail of a three-tailed insect is not.
cervical gills (cervical gill) - Gills located on the head, near the "neck," i.e. the membrane that separates the head from the thorax.
chelate - Pincer-like, having two opposable claws.
chitin - A tough natural polymer which hardens the exoskeletons of insects and serves numerous other important biological functions.
chloride epithelium (chloride epithelia) - Oval-shaped areas of modified cuticle in larval caddisflies, each surrounded by a thin sclerotized line on the venter, dorsum, or pleural regions of certain abdominal segments, involved in ion absorption.
chorion - The outer shell or covering of an insect egg.
clasper (claspers) - The claspers, also known as forceps, are a pair of appendages beneath the tip of the abdomen of male mayfly adults, which are used to grab onto the female while mating.
clypeus (clypeal) - That part of the insect head below the frons, to which the labrum is attached anteriorly.
compound eye (compound eyes) - The eyes of many arthropods, including insects and crustaceans, are composed of anywhere from 12 to 1,000 facets called ommatidia. These are called compound eyes.
conical mesonotal projection (conical mesonotal projections) - small cone shaped spike sticking up from the top and front part of the middle thorax segment.
coniform - Cone-shaped.
costa (costal) - The costa is the wing vein forming the leading (anterior) margin of an insect's wing, and the area close to it is referred to as the costal area.
costal angle (costal angles, apical angle, apical angles) - The angle of the turn in the edge of an insect's wing from the anterior, costal margin into the distal edge of the wing.
costal angulation - A bend in the front margin of the hind wing of some insects, which is used as an identifying characteristic for some mayflies.
costal process (costal processes) - A bump or point sticking up from the front margin of an insect's wing, usually the rear wing of certain mayflies. It is sometimes called a costal projection.
costal projection (costal projections) - A bump or point sticking up from the front margin of an insect's wing, usually the rear wing of certain mayflies. It is sometimes called a costal process.
costal vein (costal veins) - The thick vein which forms the front edge of an insect's wing.
coxa (coxae, coxal) - The basal segment of an arthropod's jointed leg; comes from the Latin word for "hip," and it seems more like the segment of the body to which the "actual" leg connects.
cripple (cripples) - In fly fishing, a cripple is any insect which has been injured or deformed so that it cannot escape the water. This may include stillborn emergers or fully emerged adults which have been damaged, often by wind or waves, so that they can no longer fly. Trout often favor eating crippled insects.
crochet (crochets, crocheted) - Curved hooks, typically found on the end of various insect appendages, but not used to refer to ordinary claws on legs.
crossvein (crossveins) - Short cross-wise veins in an insect wing which connect the long longitudinal (length-wise) veins.
cubital - The cubital region is the lower back corner of an insect's wing, through which the cubitus and other cubital veins run.
cubital vein (cubital veins) - Insect wing veins which stem from the cubitus.
cubitoanal crossvein (cubitoanal crossveins, cubito-anal crossvein, cubito-anal crossveins) - A short crossvein connecting the cubitus to the anal veins in the vicinity of the anal cell in a stonefly forewing.
cubitus - The fifth longitudinal vein connecting the wing margin to the wing base.
cubitus - The sixth longitudinal vein of an insect wing immediately posterior to the media (M).
dead-drift (dead-drifting) - The manner in which a fly drifts on the water when not moving by itself or by the influence of a line. Trout often prefer dead-drifting prey and imitating the dead-drift in tricky currents is a major challenge of fly fishing.
decurrent - Closely attached to and running down another body.
denticle (denticles) - Small tooth-like projects, often appearing like serrations on the tarsal claws of certain mayfly nymphs.
detritivore (detritivores) - eater of plant and animal debris
detritus - Small, loose pieces of decaying organic matter underwater.
diapause - A state of complete dormancy deeper even than hibernation. While in diapause, an organism does not move around, eat, or even grow. Some caddisfly larvae enter diapause for a few weeks to several months. Some species of microscopic zooplankton can enter diapause for several hundred years.
discal - On or relating to the disc, or upper central surface, of a body part, often used to refer to the central area of an insect wing.
discal area (discal areas) - The middle of an insect's wing.
distal (distally) - Far from the point of attachment or origin; near the tip.
dorsal (dorsum, dorsally) - Toward the top of the body.
dorsolateral - Located to the side near the dorsal (top) surface of a body or structure.
dorsoventral (dorsoventrally) - The dorsoventral axis is the axis running from the top to the bottom of the body; for example, a has a flounder is dorsoventrally flattened body shape.
dun (duns) - Mayflies have two adult stages. They first emerge from the water as duns (scientifically known as the subimago stage). They then molt into the spinner (imago) stage, in which they mate and die. Sometimes the word "dun" is confusingly used to refer to a brownish gray color in fly tying materials.
ecdysis - the process of casting the skin; moulting
eclosion (eclose) - the act of emerging from the pupal or nymphal case or hatching from the egg
elytroid - The gills on some mayfly nymphs which are enlarged as shields to protect the other gills (as on Tricorythodes and Eurylophella nymphs) are called elytroid gills.
elytron (elytra) - The hard, leathery forewing of beetles, which as a covering for the hind wings. Colloquially, this is the part that forms the main "shell" over the body when not flying.
emergence (emergences) - The transformation of a nymph or pupa into the adult winged stage of an insect. The term may refer to the emergence of an individual, or the daily or yearly event in which all individuals of a species emerge.
endemic - where found; native to; belonging exclusively to or confined to a particular place
epiproct (epiprocts) - A single appendage projecting dorsally from the tenth abdominal segment.
exserted (exsert) - Sticking out. The opposite of inserted.
femur (femora, femoral) - The main segment of an insect's leg close to the body, in between the tibia and the trochanter.
fibril (fibrils) - A fine, narrow, fibrous, structure, as in the branches of some insect gills.
filiform - Insect gills which are filamentous (feathery or thread-like) are described as filiform.
flagellomere (flagellomeres) - The individual segments of the third, typically longest, part of an insect's antenna, the flagellum.
flagellum - The third part of an insect's antennae, beyond the pedicel, often the longest and consisting of many segments called flagellomeres.
flight period (flight period) - The span of time that the adults of an adult aquatic insect species are active and flying around, in between emergence and death. It may refer to the average adult lifespan of the individuals of that species, or to the total length of time for which at least some of them are active.
forcep (forceps) - The claspers, also known as forceps, are a pair of appendages beneath the tip of the abdomen of male mayfly adults, which are used to grab onto the female while mating.
forecoxa (forecoxae) - The forecoxae are the coxae on the forelegs.
forefemur (forefemora, forefemoral) - The femur on an insect's front set of legs.
foreleg (forelegs) - The front (anterior) legs of an insect.
foretarsus (foretarsi, foretarsal) - The tarsi on the forelegs.
foretibia (foretibiae, foretibial) - The foretibiae are the tibiae on the forelegs of an insect.
foretrochantin (foretrochantins) - The trochantin on the foreleg.
fossorial - Burrowing.
friction disc (friction discs) - Oval-shaped structures formed of fine hairs or gills on the bottom of an insect's body, creating friction or suction to help them cling to rocks.
frons (frontes) - The uppermost anterior part of the head of an insect, and part of its face.
frontal shelf (frontal shelves) - An shelf-like extension at the front of the head, commonly referenced in some Heptageniidae mayflies.
frontoclypeus (frontoclypeul) - The frontoclypeus is the combined frons and clypeus when the suture between them is obsolete.
furcal pit (furcal pits) - Small, sharp depressions where the sternum of a stonefly seems to fold into itself, which might colloquially appear like an "armpit" of the insect, although it is not located immediately at the leg joint.
furcasternum (furcasternal, furcasternite) - The primary, central plate in the posterior portion of an abdominal sternum.
furcula (furculae) - A forked process. The term is typically used in reference to specific structures, which vary in location and composition across different types of insects.
gena (genae) - An insect's cheek; the side of its face.
globose - Round-shaped, like a globe.
glossa (glossae) - The inner pair of lobes at the apex of the prementum in an insect's mouth.
habitus - The physical and constitutional characteristics of an individual. Typically used meaning a view of the entire insect in an illustration or photo.
half-spent - The wing position of some insects which fall on the water after mating. Their wings on one side lay flat in the surface film while the wings on the other side stick up into the air. The term may refer to insects with their wings in that position, as well as the position itself. Many mayfly spinners fall half-spent before becoming spent.
haltere (halteres) - Flies of order Diptera have stubby club-like appendages called halteres in place of hind wings.
hammer (hammers) - A smooth clearly defined chitinous area on the ventral surface of the ninth abdominal segment of some male stoneflies used for drumming up mates.
hyaline - Highly transparent, or glassy; usually refers to insect wings, especially those of mayfly spinners.
hydrofuge - Able to shed water. Mayfly duns have hydrofuge bodies because of tiny hair-like and other structures which help trap air and shed water.
imago (imagoes) - The sexually mature adult stage of the mayfly is called the imago by scientists and the spinner by anglers.
instar (instars) - Many invertebrates molt through dozens of progressively larger and better-developed stages as they grow. Each of these stages is known as an instar. Hard-bodied nymphs typically molt through more instars than soft-bodied larvae.
intercalary (intercalaries) - Intercalaries are small veins in a mayfly's wing which are attached to the hind edge of the wing, but not to any other major vein.
intercalary vein (intercalary veins) - Intercalary veins are small veins in an insect's wing which are attached to the hind edge of the wing, but not to any other major vein. They are sometimes just called intercalaries.
irrorate - Marked with small, scattered spots or marks
labial palp (labial palpae, labial palps, labial palpal, labial palpus, labial palpi) - A segmented sensory appendage attached to the labium in an insect's mouth.
labium (labial) - The lower mouth parts of an insect; the lower lip.
labrum - The platelike structure forming the roof of the mouth of insects; the upper lip.
lamella (lamellae) - An individual plate-like or leaf-like body part, such as those forming all or part of the gills of many mayflies.
lamelliform - Insect gills which are flat and platelike are described as lamelliform.
larva (larvae) - Many classes of aquatic insects, such as caddisflies, midges, craneflies, dobsonflies, alderflies, and many more, are known as "larvae" rather than "nymphs" in their juvenile stages. They have mostly soft bodies rather than hard exoskeletons. These insects also advance through a "pupa" stage before reaching adulthood.
lateral (laterally, laterad) - A lateral position is located to the side of a body or structure, as viewed from the top.
leaf drift - The mass of dead leaves gathered on the bottom of the stream, sometimes stacked thick in still places like back eddies. Many aquatic invertebrates use the leaf drift for shelter and food. Most insects shred the leaves to digest the bacteria and plankton living on them, rather than digesting the leaves themselves.
life history (life histories) - The detailed life cycle of an organism, including the stages it passes through and characteristic behavior relating to growth and reproduction.
lobate - Having lobes.
longitudinal (longitudinally) - A longitudinal structure is aligned with the axis from the front to the back of a body or structure.
longitudinal vein (longitudinal veins) - Longitudinal veins are the major long veins running length-wise through an insect's wing, connecting the base to the outer margin, or the major branches from those veins.
lunate - Shaped like a crescent moon.
macula (maculate, maculae) - Maculae are relatively large, distinct spots or blotches on the surface of an insect's body or wing.
mandible (mandibles, mandibular) - The paired jaws of an insect which are used for grabbing food, located immediately behind the labrum.
marl - Loose sand, silt, or clay containing a high concentration of calcium carbonate.
masking hatch (masking hatches) - When two types of insects are on the water simultaneously and the trout are feeding on the less noticeable one, the more prominent insect (usually brighter and/or larger but less abundant) is known as a masking hatch.
maxilla (maxillae) - A set of paired mouth parts located behind the mandibles in most arthropods.
maxillary palp (maxillary palpae, maxillary palps, maxillary palpal, maxillary palpus, maxillary palpi) - The maxillary palp is a segmented (one to seven segments) sensory appendage attached to the maxilla of an insect's mouth.
medial - Toward the middle of the body.
median (medial, medially, mesal, mesally) - In, at, or pertaining to the middle of a body or body part.
median vein (median veins) - Longitudinal veins in an insect fore wing in between the radial and cubital veins.
mentum - The distal division of the postmentum.
mesobasisternum (mesobasisternal) - The basisternum of the mesothorax.
mesofurcasternum (mesofurcasternal) - The furcasternum of the mesothorax.
mesonotum - The top of the insect mesothorax.
mesopleuron (mesopleura, mesopleural) - The side of the insect mesothorax, and the part to which the fore wings are attached in mayflies.
mesoscutellum (mesoscutellar) - The scutellum on the mesothorax.
mesoscutum (mesoscutal) - The scutum of the mesothorax.
mesosternum (mesosterna, mesosternal) - The bottom of the insect mesothorax, to which the middle pair of legs are attached.
mesothorax (mesothoracic) - The middle segment of an insect's thorax, and in mayflies the largest.
metabasisternum (metabasisternal) - The basisternum of the metathorax.
metafurcasternum - The furcasternum of the metathorax.
metanotum (metanotal) - The top of the insect metathorax.
metapleuron (metapleurons, metapleural) - The side of the insect metathorax, to which the second pair of wings (if any) and the hind legs are attached.
metasternum (metasterna, metasternal) - The bottom of the insect metothorax, to which the hind pair of legs are attached.
metathorax (metathoracic) - The small, posterior part of the thorax.
microdrag - The imperceptibly small unnatural motions of an artificial fly on the water, caused by its connection to the line. A trout's whole life is spent watching things drift naturally, and unnatural movement too subtle for us to detect is obvious to their specialized senses.
mid-dorsal - Located longitudinally (length-wise) along the center of the body.
molt (molting) - When aquatic insects with hard exoskeletons (like mayfly and stonefly nymphs) grow bigger, their exoskeleton does not grow with them. Instead they grow a new, larger one underneath and shed the old one when it's too small. This process is called molting.
monotypic - a taxonomic level containing a single lower level. Example - a genus consisting of a single species.
morphology - The form and structure of an organism, or the study of the form and structure of organisms.
multibrooded - Producing more than one generation in a single year. Baetis mayflies are a classic example. Insects which produce a single generation with two distinct peaks (like the June and September hatches of Isonychia bicolor mayflies) are not multibrooded, because the fall insects are offspring from the previous fall instead of the current year's spring.
multivoltine - Having more than one generation per year.
naiad (naiads) - Naiad is the technical term for nymph used by modern entomologists.
natural (naturals) - A natural is a real insect (or similar creature) a trout might eat. The term is used to specify the real thing as opposed to its artificial imitation.
Nearctic - The zoogeographical region made up of North America, as far south as northern Mexico, and Greenland.
Neotropical - The zoogeographical region made up of Central and South America, the tropical southern part of Mexico, and the Caribbean.
notum - The dorsal surface of an insect's thorax or any segment of the thorax.
nymph (nymphs) - The juvenile, underwater stages of mayflies, stoneflies, dragonflies, and damselflies and other aquatic insects whose juvenile stages are covered by hard exoskeletons. The word can also refer to the fishing flies which imitate these creatures, in which case it is used as a blanket term for flies imitating any underwater stage of an invertebrate (except for crayfish and leeches).
occipital ridge (occipital ridges) - An elevated plate or seam at the back of the head. Usually concave in shape with a row of spinules (tiny spines) running along it.
occiput (occipital) - The back of the head.
ocellus (ocelli, ocellar) - A simple non-compound, single lens eye found in many arthropods. Mayflies have three ocelli in between their compound eyes. As an adjective, "ocellar" refers to the region near the ocelli.
ommatidia - The individual parts composing the compound eye.
operculate (operculum) - Lidlike; usually used to describe the pair of enlarged elytroid gills (called the operculum) which some silt-dwelling mayfly nymphs such as Caenis and Eurylophella have developed to shield their other gills from debris.
oviposit (ovipositing) - To lay eggs.
ovipositor (ovipositors) - A long, sometimes tube-like structure at the rear of female insects which helps them lay their eggs.
ovoid - Oval-shaped.
ovum (ova) - Egg.
Palearctic - The zoogeographical region made up of Eurasia north of the Himalayas, North Africa, and the temperate part of the Arabian peninsula.
palp (palpus) - A long, thin, often segmented appendage which can protrude from certain insect mouth parts such as the maxillae. Also known as the palpus.
paraglossae (paraglossa) - A paired structure on the labium at the apex of the prementum, lying on each side of the glossae.
paraproct (paraprocts) - Paired, ventral extensions of the sternite on the last abdominal segment of some aquatic insects. The term is used more broadly to describe various appendages in the anal area of some other insects.
parthenogenesis - In some species of insects and other living things, such as the mayfly species Ameletus ludens, reproduction can take place without fertilization by a male. This process is called parthenogenesis.
pectinate (pectinations) - Pectinations are tooth-like spines, arranged in rows like a comb, often found on the tarsal claws of some mayfly nymphs. Structures with pectinations are said to be pectinate.
pedicel (pedicels) - Typically refers to the second segment of an antenna, but can sometimes refer to a stalk or stem supporting an organ or other structure.
penes - The paired genital structures of most male insects, which vary widely in form and are one of the main characteristics used for species identification.
penultimate - Second-to-last.
petiole (petiolate) - A petiole is a stalk, like the one attaching a leaf to the plant stem, and organs extended on a stalk are said to be petiolate. This term is also used to describe forks in wing veins that appear at the end of a single vein (like a letter Y) instead of splitting immediately (like a V).
pharate adult (pharate adults) - Caddisflies are considered to be pupae during their transformation from larva into adult. This transformation is complete before they're ready to emerge. The emerging insect we imitate with the "pupa" patterns we tie is technically called a pharate adult. It is a fully-formed adult caddisfly with one extra layer of exoskeleton surrounding it and restricting its wings.
phylogeny (phylogenies) - The evolutionary history and development of a taxon.
piscivorous (piscivore) - Anything which eats primarily fish is a piscivore.
pleuron (pleura, pleural) - The side of the thorax of an adult insect.
polar cap (polar caps) - Coverings of additional material at at one or two poles of an insect egg (imagine the polar ice caps, if Earth were an egg).
posterior (posteriorly, posterad) - Toward the back of an organism's body. The phrase "posterior to" means "in back of."
posterolateral (posterolaterally) - Located to the side along the posterior (back) end of a body or structure.
posteromesal (posteromesally, posteromedial, posteromedially) - Located in the middle on the lateral axis, but toward the posterior end of a body or structure.
postmentum - The basal part of the labium, divided into a proximal submentum and distal mentum.
preapical - Located just before the apex.
proboscis - An extension various mouth parts of an insect into a long, thin structure, such as the apparent "tongue" of a butterfly or moth.
proleg (prolegs) - Any process or appendage that serves the purpose of a leg but does not have the exact structure and evolutionary origin of an actual leg.
pronotum - The top of the insect prothorax.
propleuron (propleura, propleural) - The side of the insect prothorax.
prosternal horn (prosternal horns) - A membranous, fingerlike projection of the prosternum in many larval caddisflies.
prosternum (prosternal) - The bottom of the insect prothorax.
prothorax (prothoracic) - The anterior (front) segment of an insect's body, to which the head is attached.
proximal (proximad, proximally) - Close to the base or point of origin; for example, on a leg or wing, the proximal parts are those closest to the body.
pterostigma - A pigmented cell or group of cells found on the wing of some insects, typically located near the wing's leading edge. It is often larger and more heavily pigmented than the surrounding cells.
pupa (pupae) - Any insect which spends most of its juvenile lifetime as a larva first becomes a pupa for a time before emerging as a fully grown adult. Depending on the species, the pupal form can be very important for fly fishermen to imitate.
quadrate - Approximately square or rectangular, although edges and corners may be slightly rounded.
radial vein (radial veins) - Major longitudinal veins on the insect fore wing in between the subcostal and median veins.
reticulation (reticulations, reticulate) - A reticulation is a pattern of fine, interlacing markings resembling a net mesh, and structures marked with this pattern are reticulate.
rostrum (rostral, rostra) - A snoutlike projection or rigid extension of the head, bearing the mouthparts at the end.
scape (scapes) - The first or basal segment of an antenna.
sclerite (sclerites, sclerotized) - A hard plate of chitinous material, such as those that form the exoskeletons of arthropods, uninterrupted by cracks or sutures.
scutellum (scutellar) - The posterior division of the notum of the mesothorax and metathorax.
scutum - The middle, often largest division of the notum on the mesothorax and metathorax.
searching pattern (searching patterns) - Any artificial fly pattern used when trout that aren't feeding selectively on anything in particular. A searching pattern may be an attractor or an imitation of something specific that the fish might favor even though it's not currently hatching.
semilunar - Shaped like a half-moon or crescent.
semioperculate - In contrast to operculate gills, which cover subsequent gills in a lid-like fashion, semioperculate gills provide only partial coverage.
sessile - Directly attached without a stem or petiole. For example, when speaking of a fork in a wing vein, a sessile fork is one that branches directly from some other vein as a fork (like in a V shape) rather than extending on a separate stem that forks in two (like a Y shape).
seta (setae, setiferous, setose, setaceous, setal) - Setae are usually small hairs on insects, although the term can sometimes refer to thicker, scaly structures of similar origin and configuration.
setal wart (setal warts) - A setiferous wart on the dorsum of the head or thorax of adult caddisflies.
shuck (shucks) - The shed exoskeleton left over when an insect molts into its next stage or instar. Most often it describes the last nymphal or pupal skin exited during emergence into a winged adult.
sinuate (sinuous, sinusoidal) - Having a smooth, wavy shape, like a sine wave.
somite (somites) - An abdominal segment.
spinner (spinner) - There are two winged stages of adult mayflies. They emerge from the water as duns, molt on land (usually) into their fully mature stage, spinners. As spinners, they mate, lay eggs, and die.
spinose (spinuous, spiniferous, spined) - Spinose structures have spines attached, whether just a few or a full covering of spines.
spiracle (spiracles) - An external tracheal opening through which some arthropods breathe.
sternacostal suture (sternacostal sutures) - The transverse division of an abdominal sternum separating the basisternum in the anterior part from the furcasternum in the posterior part.
sternite (sternites) - The bottom (ventral) part of a single segment on an insect's abdomen.
sternum (sterna, sternal) - The entire ventral (bottom) division of any part of an insect's body.
stigmatic - The stigmatic area is the part of an insect's fore wing near the tip of the front edge. Its characteristics are often important for identification.
stillborn (stillborns) - In fly fishing, a stillborn insect is one which got stuck in its nymphal or pupal shuck during emergence and floats helplessly on the surface instead of flying away. It is a specific class of cripple, although it is sometimes used interchangeably with that term.
stria (striae) - Any fine, longitudinal impressed line.
stylus (styli) - A small, pointed, process that is not articulated or jointed.
subanal plate (subanal plates) - A distinct plate-like structure near the posterior end of an insect's abdomen, this term can mean different things (i.e. parts of different abdominal segments) for different insects.
subapical - Located near, but not quite at, the apex.
subcostal vein (subcostal veins) - The first major longitudinal vein after the costal (front edge) vein and before the first radial vein.
subcylindrical - Almost but not exactly cylindrical.
subequal - Almost but not exactly equal.
subimago (subimagoes) - Mayfly nymphs emerge from the water into subimagoes, better known to anglers as "duns." They are a sexually immature, winged, recognizably adult stage and they must molt one more time into imagoes or "spinners" before they can mate.
submentum - The basal division of the postmentum.
suboccipital - Situated under, or posterior to, the occiput.
subparallel - Almost or approximately parallel.
subtriangular - Almost but not quite triangular, being somewhat rounded at the corners or curved on the edges instead.
suctorial - Adapted for sucking.
suture (sutures) - A groove marking the line of fusion of two formerly distinct plates of an arthropod's exoskeleton.
synonym (synonyms) - The former name of a taxon, usually a species. Entomologists frequently discover that two insects originally described as different species are actually the same, and they drop one of the names. The dropped name is said to be a synonym of the remaining name.
tarsal claw (tarsal claws) - The claws at the tip of the tarsus, on an insect's "foot."
tarsus (tarsi, tarsal) - The often multi-segmented outer leg section of an insect, which attached to the tibia. Colloquially, it's the closest thing an insect has to a "foot."
tergite (tergites) - The top (dorsal) part of a single segment on an insect's abdomen when it consists of a single chitinous plate (sclerite), or an individual sclerite if the segment has more than one.
tergum (terga) - The dorsal part of an abdominal segment. Also used to describe the entire abdominal dorsum or the thoracic dorsal segments of Odonata.
terminal filament (terminal filaments) - The technical term for the middle tail of a 3-tailed insect (the outer tails being called cerci).
terminal segment (terminal segments) - The outermost part of a segmented structure, i.e. the segment forming its tip.
terrestrial (terrestrials) - Insects which live on land and are fed on by trout only when they incidentally fall into the water are known as "terrestrials" to fly anglers, and they're very important in late summer.
thorax (thoracic) - The thorax is the middle part of an insect's body, in between the abdomen and the head, and to which the legs and wings are attached.
tibia (tibiae, tibial) - A middle segments in the leg of an insect, located between the femur and the tarsus.
titillator (titillators) - Spines, small plates, or slender processes at distal extremity of aedeagus or penes of an insect.
trachea (tracheae, tracheal, tracheated) - Tracheae are internal air tubes, like those in the gills or the spiracles of an insect's respiratory system. Structures containing tracheae may be described as tracheal.
transverse - Situated or extending across something from side to side, as opposed to longitudinally.
trivoltine - Producing three generations per year.
trochanter (trochanters) - A short segment near the base of an insect's leg, which joins the coxa on the inside to the femur on the outside.
truncate - Cut off. This is often used to describe the square appearance of the gills of Maccaffertium mayfly nymphs, for example, as opposed to the pointed gills of the closely related genus Stenacron.
tubercle (tubercles) - Various peculiar little bumps or projections on an insect. Their character is important for the identification of many kinds of insects, such as the nymphs of Ephemerellidae mayflies.
turbinate - Shaped like a top or elevated on a stalk; usually refers to the eyes of some adult male Baetidae mayflies which are wider near the tip than at the base.
univoltine - Producing more than one generation per year.
veinlet (veinlets) - Short insect wing veins connecting the major longitudinal veins to the wing margin.
venation - The pattern in which the veins on the wings of an insect are arranged. It is usually one of the most useful identifying characteristics.
ventral (ventrally, venter) - Toward or on the bottom.
ventroapical - Located on the ventral side of a structure near its apex.
ventrolateral (ventrolaterally) - Located to the side of the lower part of a body or structure.
vertex (vertices) - In entomology, vertex refers to the top of an insect's head.
vestigial (vestige, vestiges) - An anatomical structure which has become dysfunctional or reduced after hundreds of generations of non-use is vestigial, and whatever remains of it is a vestige. The human tailbone is a classic example.
whorl (whorls) - A pattern of spirals or concentric circles, or structures arranged in this pattern.
wing articulation (wing articulations) - The point at which a wing joins the body.
wing pad (wing pads) - A protrusion from the thorax of an insect nymph which holds the developing wings. Black wing pads usually indicate that the nymph is nearly ready to emerge into an adult.
zoogeographical (zoogeographic) - Zoogeographicals regions are large (continent-scale or larger) geographic regions within which animal communities are more similar and closely related than they are among different regions.