Review Sheet -- Test 3 (Week 11) Biology 1224 -- Entomology; James Adams
Sense Organs -- (Chapter 6, pgs. 137-148); remember, check the Entomological Terms
handout. Simple sense organs are called sensilla. Will do a central nervous system over-
view at the beginning of the behavior system later.
Compound -- composed of several (# 8 in Collembolans) to thousands (up to 28,000 in
adult odonates) of separate units called ommatidia; each ommatidium has its own corneal lens,
typically hexagonal, on the surface of the eye.
Simple -- single round corneal lens: 1) ocelli (dorsal) are located on top of the head typically
in a triangle; found together with compound eyes in adult stages of most insects and nymphs of
exopterygotes; 2) stemmata (lateral ocelli) when present are often more numerous than dorsal ocelli,
numbering up to a dozen on each side; these are found in most endopterygote larvae, not just larvae
of Lepidoptera; endopterygote larvae also typically lack compound eyes, except some Mecopteran
larvae. Strepsipteran adults have stemmata; flea adults have simple eyes if any; other parasitic types
and cave-dwelling insects often lack eyes as well. Don't forget that Protura and Diplura lack eyes.
Simple ocelli do not allow for much more than recognition of light and dark patterns
around the insect, though they do respond strongly to changes in the visual field around them
(indicating movement -- potential predators), and probably help maintain steady flight by helping
the insect maintain a relatively constant visual horizon. Stemmata, on the other hand, do allow for
image formation; inputs from separate stemmata can be put together, similar to a compound eye.
Image formation by compound eyes, on the other hand, is extremely complex, involving
separate inputs from each ommatidium that are put together into a coherent whole by the brain. The
resolving power of even the best insect eye (odonates) would seem to be significantly less than that
of a vertebrate eye, however, though the actual responses to small moving items in their field of view
would suggest otherwise. Insect eyes are sensitive to color, with many insects being able to see a
broader spectrum of light than we are, though in a number insects the red end of the spectrum is
restricted or lost. Many insects have the ability to detect ultraviolet light -- this is important as many
flowers have ultraviolet "targets" indicating where nectar is, and some insects also have ultraviolet
patterns on the wing for attracting mates. Butterflies seem to have the broadest visual spectrum, from
the ultraviolet through the red (for seeing red flowers, wing patterns). Some species of Hymenoptera
are even responsive to polarized light, which allows the insects to determine direction by seeing light
rays, even on a day when there is but a small patch of blue sky visible. Night flying insects (especially
Leps, Trichops., Coleops., Neurops) have extra tracheae (which are shiny) in the eyes, forming
a tapetum (reflective surface) that allows them a greater chance of processing dim light at night.
Mechanoreception and Hearing
Tactile (touch) sensation involves bending of hairs (setae) and receptors that respond to
bending of the exoskeleton, in other words, to mechanical (pressure) changes. Stretch
receptors (remember molting) also respond to changes in shape of tissues.
Hearing is, in essence, mechanoreception as well, as a hearing organ responds to pressure
changes in the air. The hearing organ, typically called a tympanum, consists of a thin membrane
stretched across an open cavity that can freely vibrate when struck by sound. A moderate number
of insects cannot hear, but many, many can. Some insects which need to detect sound are obvious,
such as orthopterans and cicadas that attract mates with sound. The tympanal organs are found in
various places on the body: on the abdomen of cicadas/grasshoppers and some moths, on the legs
of crickets/katydids, mesothorax of some hemips, and metathorax of other moths. In the night flying
insects, the tympanum allows detection of ultrasonic calls of bats, important if you want to avoid being
eaten! Some other receptors (including hairs on various parts of the body, including the antennae
[mosquitos]) can detect sound as well, through vibrations in wood, in the ground, or in the air --
remember drumming of stoneflies, for instance.
Chemoreception (including Olfaction [Smell] and Taste)
Chemoreceptors are typically sensillae (setae) that respond to airborne (smell) or dissolved
chemicals (taste). The main body parts that have chemoreceptors in different insects include:
antennae, mouthparts, legs/"feet" (specifically the tarsal and pretarsal segments), and the ovipositor.
Hopefully it is obvious that every insect has to have at least some chemoreceptive capabilities.
An IN CLASS exercise!
Explain what types of chemical cues each body part listed above would be useful in detecting:
Antennae (detect airborne and dissolved chemicals -- smell and taste):
Ovipositor (technically speaking taste as well):