Download Insect Morphology

Aquatic Insect Morphology
There are three main body regions; namely, the head, thorax, and the abdomen. The head
includes the eyes, antennae, and mouthparts; the thorax bears the legs and wingpads; and
the abdomen includes the gills and three terminal “tails”. Keep in mind when you think
about insect morphology that all of these insect parts may be highly modified between
groups, and it is important to recognize the position relative to other parts because they
may not always look like you would expect them to look.
Head
A pair of large compound eyes is located posterior-dorsally. Mesad to the eyes are three
ocelli (singular= ocellus), two lateral and one median. Like the eyes, the ocelli are light
receptors; however, ocelli do not produce clear images. The filiform (=threadlike)
antenna (singular= antenna) are positioned anterior to the eyes.
Ventrally, the head region is dominated by the mouthparts, consisting of the labrum,
mandibles, hypopharynx, maxillae, and labium. If you place an insect with the ventral
side facing up, the labium will be on top. It is the most ventral of the mouthparts. The
paired lateral labial palpi are conspicuous. Mesad to the palpi, along the anterior margin
of the labium are four small projections—a medial pair of glossi (singular= glossus) and a
lateral pair of paraglossi (singular= paraglossus). The paired maxillae are located dorsal
(and maybe a bit lateral) to the labium. Like the labium, each maxilla includes a labial
palp, the maxillary palpus. The labium and the maxillae (and also the labrum) are
important in food handling and bear sensory structures (i.e. chemoreceptors and
mechanoreceptors). The paired mandibles are the next structures encountered. The
toothed and ridged surfaces are used in procuring food (e.g., scraping) and in food
processing (e.g., smashing and shredding). The median labrum resembles a hinged upper
lip. Its proximal edge is located near the anterior border of the head, which it joins via
the clypus (a sclerites or plate that in this mayfly is fused to the rest of the head capsule).
Nestled between the paired mouthparts (mandibles and maxillae) look for the
hypopharynx a median (a median, bump-like structure).
Thorax
The thorax contains three segments—the prothorax, the mesothorax and the metathorax.
Each segment bears a pair of legs and both of the posterior two segments have caudally
projecting wing pads (called the forewing pad and the hindwing pad, respectively). The
metathorax in mayflies is mostly covered by the mesothorax.
With a bit of imagination, individual segments of the thorax (and abdomen) resemble
boxes open on each end; that is, they have dorsal, ventral, and two lateral surfaces. The
dorsal surface is the notum (= tergum when we refer to them in the abdomen), the the
ventral plate is the sternum, and the two lateral sides are the pleura (singular= pleuron).
To denose a particular segment of the thorax, these terms can be combined with the
appropriate suffix (pro-, mexo-, meta-), producing such terms as prothorax, metapleuron,
and mesosternum. The thorax may also bear gills in some orders of aquatic insects.
Each of the six legs consists of the same body segments. The basal segment (closest to
the body) is the coxa, which attaches the rest of the leg to the body wall. The trocanter is
a small segment distal to the coxa. Next comes the femur (often recognized as a broad,
somewhat enlarged segment), the tibia, and finally the tarsus (which may be made of up
to five tarsal segments), which may bear one or more claws.
Abdomen
The abdomen consists of ten segments. When keying aquatic insects, you will need to
accurately count and identify segments. The numbering system begins anterioraly (from
where the abdomen connects to the thorax as segment 1). However, when counting
segments, it works better to begin posteriorly and count backward, because in some cases
the thorax covers and obscures the first segment(s). The tenth segment is the one bearing
three caudal filaments (= “tails”).
Gills are attached laterially to many of the abdominal segments. The location and
number of segments with gills are characters used in keying. Within a gill plate,
branching trachea are visible. The trachea are part of a network of tubes that transport
gasses throughout the body. Gills may also appear at the end of the abdomen formed as
lancolate, platy projections in a group of three.
Posteriorly, three caudal filaments project from the abdomen. These are easily broken on
preserved specimen, so be careful when relying on this as the only character for
identification. The lateral, symmetrical “tails” are cerci (singular= cercus), and the
central median filament is the terminal filament. Cerci or terminal filaments may be
present or absent on aquatic insects.