Review Sheet -- Test 3 (Week 11) Biology 1224 -- Entomology; James Adams
Internal Anatomy -- Now that we "know" the orders, time to
discuss internal anatomy
including similarities/differences between orders. REVISIT your Entomological Terms sheet.
Digestive System --
(Chap. 5, pgs. 93-99; see Figure 5.2; scan the nutrition section, pgs.
Digestion means the break down of food into simple molecules which are then absorbed;
digestion can be either physical (mechanical -- teeth, for example) or chemical (enzymatic).
Three main sections: foregut (stomaodaeum), midgut (mesenteron), hindgut (proctodaeum).
Both fore- and hindgut are lined with a thin cuticle (continuous with outer cuticle); the midgut is
lined with a delicate absorptive epithelium, which is protected by a secreted peritrophic
membrane. Attached at mid-/hindgut junction are the Malpighian tubules (main organ of the
excretory system); both digestive and internal wastes are therefore released from the digestive
system. The foregut includes the muscular pharynx ("throat") involved in intake, the esophagus
which is a passageway to the crop (a storage sac). Release of food from the crop is controlled
by the muscular proventriculus. This sclerotized section may have complex teeth or spines,
which helps grind up the food, and may also allow enzymes forward from midgut. Fluid feeders
typically have a smooth proventriculus that simply acts as a sphincter. The midgut secretes
digestive enzymes, and certain cells with microvilli are involved in absorption of digested nutrients.
In many wood feeding species in several orders, the midgut has side pouches that house symbiotic
bacteria that aid in wood digestion. The hindgut includes the anterior intestine (ileum), the colon,
and the rectum. As it is lined with cuticle, much of the lining is impermeable, but the rectal pads
are active in moving water, salts, sugars and amino acids back into the body cavity, so they are
not only involved in nutrition, but also salt and water balance, an excretory system function.
Modifications of this general form are common. Fluid feeders typically have side pouches
that aid in fluid processing/absorption, and may have (as indicated when discussing Hemiptera)
microscopic symbiotes to produce added nutrients from the nutritionally incomplete fluid meal.
The midgut is blind-ended in larval Neuroptera and some larval Hymenoptera. The ileum has
more bacterial symbiotes in termites and some larval scarabs. The rectum in some aquatic larvae
(odonates) has internalized gills aiding in oxygen uptake.
Digestion can occur in a variety of places -- for example, many species in the Reduviidae
inject digestive enzymes into prey so they can intake a more liquid meal; houseflies exude
enzymes onto the surface of foods to partially liquify the food. Salivary glands produce some
enzymes, which may continue to help with digestion on into the crop -- this is the main digestion
that occurs in Orthoptera and Blattodea; and the proventriculus, as mentioned above, may have
"teeth" to help physically break up the food.
The peritrophic membrane, used for protection of the midgut, is a fibrous network that
includes chitin, glycoproteins and proteins; it basically functions like mucus in vertebrates.
Enzymes may pass into the gut cavity, and digested materials can be selectively absorbed through
tiny pores in the peritrophic membrane. Once absorbed, the nutrients circulate through the
hemolymph (see below) to body organs and some is stored in fat body. Some infectious agents
can gain entry into the body here.
Movement of food through the gut is by peristalsis, involving circular and longitudinal
muscles in the walls of the gut. Very few sensory nerves are associated with the gut; most sensory
nerves are at the front end. What this indicates is that, once found, the appropriate food is eaten,
then simply processed by the rest of the digestive system.
Respiratory System --
(Chapter 5, pgs. 100-107)
The insect respiratory system is a tracheal system, with openings to the outside called
spiracles. The tracheal system is an air-filled system of passageways, with branches (tracheoles)
that are in contact with a good percentage of the stationary cells in the body, especially in the
muscles of flying adults. The number of tracheoles number in excess of one million in a moderate
sized insect. Blood and hemoglobin (which carries oxygen in vertebrates) are typically not used
in most insects, though there are a few exceptions (Chironomidae, etc.); as such, there is no close
association between the circulatory and respiratory systems in hexapods. A few small species
(some Protura/Collembola) that favor damp places may use thin body surface cuticle for exchange;
but the body surface lacks sufficient surface area and is impermeable in most hexapod species --
the tracheal system solves both these problems -- lots of surface area and direct oxygen delivery.
Diffusion is the dominant moving force for both oxygen and carbon dioxide; diffusion is
much faster through air than tissue, which is the reason for the incredibly branching tracheal
system. The cytoplasm of the target cells is the only place where diffusion is taking place through
liquid. Still, there is a limit in size at which diffusion will work, and this is likely the main reason
why there are no giant insects. Many larger, very active species have muscular sacks that can
ventilate the system (move air through faster), and activity of the body also aids in air movement.
Pairs of spiracles are typically found on some thoracic and abdominal segments, with many
variations in different species (such as single end-of-abdomen spiracles in some aquatic and
subdermal parasitic larvae). Spiracles can typically be closed temporarily when necessary (for
instance, to reduce water loss or prevent water from entering when submerged), but, of course,
there is a limit to the amount time the spiracles can remain closed. Spiracles often have hairs
around/in the openings to help filter "stuff" out of the air.
The lining of the larger tracheae is shed with each molt.
Exchange in aquatic insects varies, and may involve various gills. Additionally, some
species, particularly aquatic beetles and bugs, have specialized hairs that allow them to visit the
surface, pick up air, and carry a bubble of air underwater with them (a plastron gill). Some carry
extra air under the elytra (remember that the abdominal tergites in beetles are often quite thin).
The concentrations of gases initially equal that of the air, but as the oxygen is depleted, more
oxygen diffuses out of the water into the bubble for use.
Circulatory System --
(Chapter 5, pgs.107-110; see Figure 5.7)
An open circulatory system is typically found in insects. The heart is located dorsally in
the abdomen, with openings, or ostia, into the abdominal cavity. The hemolymph enters the
heart through each ostium, and is pumped forward through the main vessel (aorta) into the head,
where the hemolymph is dumped into the interstitial spaces and flows freely around the body
tissues through the hemocoel. The appendages may have extra "pumps" at the base to direct
hemolymph into the appendage -- the legs in particular have these pumps, and are additionally
divided by a septum to direct blood flow down one side and up the other. The open system is a
low pressure system, and works fine for delivery of nutrients, hormones, etc. and waste removal.
Remember, there is no need for high pressure since oxygen delivery/carbon dioxide removal are
independent of the circulatory system.
The cuticle, including that of the fore- and hindgut and also the tracheal system, is the
main barrier for pathogens. If pathogens do enter the body, there are circulating phagocytes and
antibacterial/antiviral molecules in the hemolymph. Larger parasites may be encapsulated by
other circulating cells. However, there are a number of pathogens and large parasites which are
typically fatal if they enter the body. Clotting mechanisms exist in some species, but the low
pressure system and hard exoskeleton make clotting relatively unimportant, and holes in the
exoskeleton may be repaired in immatures upon molting into the next instar.
Excretory System --
(Chapter 5, pgs. 110-112; see Figure 5.8a)
The main wastes generated in the body are nitrogenous (similar to vertebrates) from the
metabolism of proteins. Also handled by the excretory system are excess salts or water. The
main organs involved are the Malpighian tubules that are freely movable in the hemocoel of
the abdomen and the rectal pads (see above). The process is relatively simple, with nitrogenous
wastes (mainly uric acid), excess salts, and, oddly, some nutrients as well being actively removed
from the hemolymph and deposited in the Malpighian tubules. This material then moves into the
hindgut (at the junction where the Malpighian tubules are attached to the gut), the nutrients reab-
sorbed in most species through the rectal pads, and the wastes and excess materials allowed to
pass to the outside. Ion pumps associated with the rectum are particularly important in also
drawing water back into the body, thus conserving water for active terrestrial species. For those
species with a sugar rich fluid diet (sap/nectar feeders), the opposite is often the problem; these
must release the excess fluids and even sugar, so the fecal material is often quite liquid (think of
the honeydew of aphids). The fat body can also store excess wastes, and the white uric acid can
be used as a pigment in the wings/exoskeleton.
The number of Malpighian tubules is variable across orders. A few species lack them
altogether, others have a few in multiples of two, and some orders (Ephemeroptera, some
orthopteroids, and Hymenoptera) may have 50 or more.
Reproductive System --
(Chapter 4, pgs. 74-78; see Figure 4.7)
Male -- testes produce the sperm; pass through the vas deferens to the seminal vesicles,
where sperm may be stored for a little while; the ejaculatory duct exits the seminal vesicles and
unite forming a single duct; sperm passing through this duct then leaves the body through the
aedeagus, which is often quite sclerotized and highly modified in shape depending on the species.
Accessory glands are almost always present, and particularly well developed in species that
construct a spermatophore.
Female -- ovaries, which may be single or divided into multiple egg-containing ovarioles;
eggs pass from here into a common oviduct and then into the genital chamber, which has attached
accessory glands and the spermatheca (where sperm may be stored); fertilization takes place in
the genital chamber. Once fertilized, eggs are laid with an ovipositor, which may be significantly
modified in one way or another in different species.
Both systems may be modified to an extent, such as separate mating and oviposition
openings in the females of ditrysian Lepidoptera. The external openings to the reproductive
systems typically involve abdominal segments 8 and/or 9 (and/or 10). One general concept:
whatever modification you see in the reproductive system of one sex will be accompanied by a
matching modification of the other sex (in other words, the parts must "fit").