Digestive strategies in insects

In the chapter "How successful are insects?" we have noted that insects present a much greater biodiversity than vertebrates. Among insects, the number of the known species of beetles (insect order Coleoptera) alone is about seven times the number of all the vertebrate species put together. The main factors, responsible for so extensive biodiversity presented by insects, are: (i) their great dispersal capacity due to presence of wings, (ii) their highly developed fecundity; a single female laying some hundred to some thousand eggs, and (iii) their remarkable digestive adaptability to different types of food. The last mentioned property allows insects to adapt themselves to different sources of food, and thus diversify to exploit different trophic niches.

A large number of insects are plant feeders or phytophagous. They feed on leaves, buds, flower nectaries, pollen, stem and roots. Some insects are wood borers, attacking healthy and rotting wood. Some insect groups have organs of feeding in the form of feeding needles, and they suck plant sap. Stored grains and other forms of stored food products are infested by many insects. Numerous insect species are carnivorous and predaceous. Some insects groups are parasitic on other insects. In entomological jargon they are referred to as entomophagous insects. Among insects are external parasites of birds and mammals. Mosquitoes and the tse-tse fly (Glossina) are blood suckers. Saprophagous insects (that is insects feeding on dead organic matter) are important among decomposers (that is those organisms which consume organic debris in soil and water, and release nutrients for plants). Among insects some are feeders of fibrous proteins (wool, keratin, silk etc.); they are pests of carpets, clothes, furs and museum specimens. Termites and book lice attack libraries and book stores, as they are efficient, thanks to their symbionts, in digesting and assimilating cellulose.

A few words about the infamous blood sucking habit of insects. Blood sucking in some cases of various tropical moths is an adaptation from fruit feeding to sucking on the secretions of the eyes of mammals and finally blood from small vessels. Blood sucking among the fleas probably dates from the Mesozoic, even if the pre-fleas, of questionable origin, were possibly parasites of pterosaurians. Real fleas are found from the Eocene (Rasnitsyn and Quicke, 2002). Blood sucking is very old among the tse-tse flies (Oligocene) and Culicidae (Cretaceous).

Let us first become familiar with the different parts of the insect digestive system. Between the bases of the mouth appendages, referred to as mouth parts in insect anatomy, is located the mouth. Mouth opens into a small buccal chamber, followed by a narrow tubular oesophagus. Opening into the buccal chamber are the ducts of a pair of salivary glands. The oesophagus enlarges posteriorly to form a sac like region, the crop, which is meant for storage of ingested food. In those insects, which feed on solid food, e.g. cockroach, the crop is connected behind with a globular and muscular part of the gut, called the proventriculus or the gizzard, which crushes the food and reduces its particle size to help digestion. The proventriculus leads behind into the mid-gut or the mesenteron, which is generally a soft simple tube, but in many insects it is differentiated into certain regions. Arising from the anterior end of the mid-gut are a variable number of pouches or diverticula, called the mesenteric or gastric caeca. Digestive enzymes are mostly secreted by the glandular epithelial lining of the mesenteron and of the caeca. The mid-gut presents a special feature, a membranous tube within it, preventing food particles from coming in direct contact with the epithelial lining of this part. This membranous tube is called the peritrophic membrane. It is believed to protect the epithelium of the mid-gut from abrasion, as does the mucus in the digestive system of vertebrates. The mesenteron is followed by the last part of the digestive tube, the hind-gut, which is a tubular region leading from the mesenteron to the anus. Generally the hind-gut is differentiated into an anterior simple tubular portion, the anterior intestine, and a dilated last part of the hind-gut, the posterior intestine or the rectum. While most of digestion and absorption of the digested food take place in the mesenteron, absorption, mainly of water, occurs in the hind-gut.

At the junction of the mid-gut and the hind-gut arise a variable number of tubular structures, opening into the gut, and with closed ends away from the gut. They are urine forming organs, and are called the Malpighian tubules. They discharge the excretory fluid, secreted by their glandular epithelial wall, into the first part of the hind-gut. In the hind-gut most water in the urine is absorbed, and thus conservation of water is brought about, and the excreta in a highly concentrated or solid form is voided along with feces. This is an account of the digestive organs in a typical insect; several variations are seen in different insect groups.

While the chief secretory part of the insect alimentary tract is the mesenteron with its gastric caeca, secretion is also the function of the paired salivary glands, the ducts of which open into the buccal chamber. The salivary secretion moistens the food to help mastication by the mandibles among the mouth parts, and also lubricates the food to help its posteriorly movement. But the latter effect disappears by the time the food reaches the mesenteron, as in the cockroach Blatella. In some insects some digestive enzymes may be present in the salivary fluid.

Now let us turn to the enzymatic part of digestion. The three broad categories of enzymes, which occur in the digestive system of vertebrates, are present in insects too. They are protein digesting enzymes or proteases, carbohydrate digesting enzymes or carbohydrases, and fat digesting enzymes or lipases. In omnivorous insects, like cockroaches, the mesenteric secretion contains maltase, invertase and lactase among carbohydrases, trypsin and erepsin among proteases, and lipases. But exclusively predaceous and carrion feeding insects have mainly proteases and lipases in their mid-gut secretion. In blood feeders, like the tse-tse fly (Glossina), there are special proteases for digestion of haemoglobin. In exclusively phytophagous insects carbohydrases predominate in the me-senteric secretion. Insects feeding exclusively on nectaries of flowers have an abundance of invertases, which are carbohydrases for digesting disaccharides into simple sugars, in their mid-gut. The carnivorous larva of the blow-fly Lucilia, which burrows deep into the body of sheep, causing grave injury, has a special protease, called collagenase, which digests the proteins callagen and elastin, present in connective tissues of sheep. Wood boring larvae of the beetles, belonging to the families Anobiidae, Buprestidae and Cerambycidae, have a special carbohydrate digesting enzyme, called cellulase, to digest cellulose present in abundance in wood. The cattle grub (Hypoderma), the larvae of which penetrate through tissues of cattle, has a special carbohydrase, glycogenase in the larval mid-gut, in addition to the proteases trypsin and erepsin and also lipase. Glycogenase helps digestion of the animal starch or glycogen. Thus the nature of the digestive enzymes present in an insect are related to the food the insect consumes, and, as the food of different insects varies very much, there are corresponding variations in the enzyme contents of the digestive system.

As has been pointed out above, some wood boring insects have cellulase in their mid-gut secretion. If we make an extract of the mesenteron wall of a larva of a long horned beetle (Family Cerambycidae) and try to filter it, using a filter paper disc, a hole is made in the disc. This is due to cellulase action on the cellulose fibres in the filter paper. Most phytophagous insects, however, do not have cellulase in their digestive secretion. Plant cells have a cellulose wall around them. Hence, before the nutrients within the cell are released and made available for digestion, the cellulose cell wall should be broken. This is done by the mechanical action of the mandibles among the mouth parts and of the proventriculus. But 100% cells in the plant food do not get ruptured this way. Many cells remain unbroken, and come out intact, with their organization unaffected, with feces.

Many insects digest cellulose in their food with the help of symbiotic bacteria and Protozoa. Such insects are Rhagium (Coleoptera), Tipula (Diptera), the cockroach Cryptocercus and termites. In termites, cellulose digestion occurs in the hind-gut. A part of the hind-gut is dilated, and lodges a number of different flagellate Protozoa. The flagellates ingest pieces of cellulose and digest them. The termite host gets nourishment by digesting dead flagellates and by absorbing their secretion. The symbiotic Protozoa make about 1 /3rd of the weight of the nymphs of Zootermopsis (Day and Waterhouse, 1953). When in an experiment the flagellates are removed from the gut of termites, the insects are unable to digest cellulose. When refaunated with the Protozoa, they are again able to live on a diet rich in cellulose. Dung beetles (Family Scarabaeidae) also feed on cellulose rich diet. In these insects, too, a part of the hind-gut is dilated to form a fermentation chamber. The cuticular lining of the chamber forms a number of branching spines. The cuticle is specially thin between the bases of these spines, and is provided with fine canal like gaps (Wiggles-worth, 1953). Pieces of cellulose are held among the spines for days together, and are digested by cellulase produced by the cellulose fermenting bacteria, which abound in the fermentation chamber.

Some insects take help of fungi to digest cellulose rich material. But this is done outside the insect body. Many ants and termites maintain "fungus gardens" inside their nest. The fungus in the gardens grows on a cellulose substrate (bits of leaves in case of parasol ants; see the chapter "Parasol ants" for details), and the fungus is the immediate source of nourishment for these insects.

Larvae of the wax moth (GallerĂ­a) tunnel through the hive of the honey bee, and feed on wax. Digestion of the wax is also believed to be brought about with the help of symbiotic bacteria.

Fibrous proteins are another category of materials difficult to digest. Clothes moth (Tinea) feeds on woolen clothes and carpets. Larvae of this moth are able to digest about 47% of wool ingested. The proteases in the mid-gut of this insect require a very high pH (about 9.5) for their action, but they are not able to break disulphide bonds (S-S) in the wool proteins. However, the high value of pH, existing in the mid-gut of the insect does the job; it breaks the disulphide bonds. The resulting smaller length polymers are then readily digested by the mesenteron proteases.

The mid-gut is typically a simple tube, undifferentiated into further regions. The mid-gut caeca serve to extend the area of its secretory and absorptive epithelium. But in some insects there are some functional divisions of the mid-gut. In the larva of the mosquito Aedes there is an anterior portion of the mid-gut, in which digestion and absorption chiefly of fats take place, while in second half those of glycogen. In the adult of the blood sucking tse-tse fly (Glossina) three regions may be made out in the long and coiled mid-gut. The first half of the mesenteron is a little broader tube, in which no enzymes are present, and only absorption of water takes place to thicken the ingested blood. This is followed by a middle region with deeply staining and enzyme secreting epithelial cells. In this region digestion mainly occurs. This second region continues behind into a long narrow tubular region, which is mainly for absorption of digested food. In the tortoise beetle Aspidomorpha miliaris the mid-gut is divided into a broad sac like anterior region, and a narrow tubular and coiled posterior region (Shrivastava and Verma, 1982). The two regions have been referred to by the authors as MS1 and MS2. The peritrophic membrane of the MS1 presents a special feature, namely presence of certain cells in its thickness, the peritrophic membrane cells. These cells are actually mesenteric epithelial cells, which have left the epithelium and have come to lie in the thickness of the forming peritrophic membrane. No such cells are present in the peritrophic membrane of MS2. The peritrophic membrane cells disintegrate releasing a cluster of tiny globules, seemingly secretory globules which pass into the lumen bound by the peritrophic membrane (endoperitrophic lumen) of MS1. Verma and Shrivastava (1989) have recorded a differential distribution of enzymes in the mid-gut of this tortoise beetle. They have noted protease and lipase activity specially pronounced in the endoperitrophic lumen of MS1, amylase (a carbohydrase) activity in the ectoperitrophic lumen of MS1, and lipase and amylase activity in MS2. It appears that the specially high protease activity in the endoperitrophic space of MS1 is due to the peritrophic membrane cells, and that it serves to dissolve away the cytoplasmic cover around the cell inclusions in the leaf cells, and then the released starch grains and lipid globules move on to the extraperitrophic lumen of MS1 and to MS2 for their digestion.

Mid-gut is the main producer of digestive enzymes. But in some insects enzymes may be detected in the crop. This is due to the mesenteric enzymes moving forward as a result of antiperistalsis, i.e. peristaltic contractions moving from behind forward. Some digestive enzymes may be present in the salivary gland secretion. In many insects the salivary secretion is a watery fluid without any enzymes, and is meant to moisten the food during mandibular action. But in some insects the salivary fluid includes some enzymes. In the leaf hopper Empoasca, which sucks plant sap, the salivary secretion contains an amylase to dissolve starch grains in situ in plant cells. In plant feeding insects amylase and invertase (both carbohydrases) are commonly present in salivary fluid. In blood sucking insects the salivary fluid includes anticoagulin to prevent clotting of blood.

As has been pointed out in the preceding paragraph, the amylase in salivary fluid of plant sap feeding insects is meant to bring about digestion of starch grains outside the digestive system. Such extraintestinal digestion is known in some other insects too. In the larvae of some predatory ground beetles (Family Carabidae) and in carrion feeding Panorpa (Family Panorpidae of Order Mecoptera) digestion is largely extraintestinal, as during feeding mesenteric secretion is regurgitated, and digestion takes place outside the buccal cavity. The larva of diving beetles (Family Dytiscidae) is very active and predaceous. Its long sickle like mandibles are nearly tubular. They are made to pierce the body of the prey, and then a fluid is seen moving down the mandibular canal, and getting injected into the prey's body. One can see under some magnification that, following the injection of the fluid, which is actually the mesenteric secretion, the tissues of the prey are getting liquefied in its body, and getting sucked up through the mandibular lumen into the body of the larva. In Oedemerid beetles which feed on pollen grains, the pollen grains germinate in the gut and pollen tubes grow slightly before being digested (Arnett, 1962). Other pollen-eating beetles crack the cuticles of pollen grains with the mandibles.

These diverse digestion strategies among insects have been an important factor in development and evolution of the huge biodiversity they present. This situation is well illustrated by the case of the species complex of the tree hopper Enchenopa binotata, discussed by Rodriguez et al. (2004). The species in the complex are obviously closely related, living on different host plants, in process of diversification, and found in the same geographical area. It has been inferred that in a common geographical area members of one species, due to small changes in their digestive physiology, have moved on to different hosts. There are differences in such characteristics of the different host plants as length of flowering period, time and extent of autumnal shedding of leaves etc., and this has led to developing different timings for the tree hopper's life history, such as mating and egg laying time, length of larval period, diapause time etc. This has brought about some reproductive isolation among the tree hoppers population, living on different host plants. As a result of this isolation different patterns of vibrational communication between prospective mates have developed, the reproductive isolation has been strengthened, and diversification has gone ahead. How a single point mutation in the genome of an insect species, resulting in replacement of one amino acid molecule with another in a protein structure, permits the insect to invade a new host plant is well demonstrated by the studies by Labeyrie and Dobler (2004) on species of Chrysochus. This work has been cited at some length in the chapter "Chemical defence in leaf beetles."

References

Arnett, R. H. 1962. The tarnished beetles. J. Wash. Acad. Sci. 52: 9-15.

Day, M. F. and Waterhouse, D. F., 1953. The mechanism of digestion. In: Insect

Physiology (Editor IK D. Roeder). John Wiley and Sons, New York. Labeyrie, E. and Dobler, S., 2004. Molecular adaptation of Chrysochus leaf beetles to toxic compounds in their food plants. Molecular Biology and Evolution 21 (2): 218-221.

Rodriguez, R. L., Sullivan, L. E. and Cocroft, B. 2004. Vibrational communication and reproductive isolation in the Enchenopa binotata species complex of tree hoppers (Hemiptera, Membracidae). Evolution 58 (3): 571-578. Rasnitsyn, A. P. and Quicke, D. L. J. 2002. History of Insects. Kluwer Academic

Publs., Dordrecht and Boston. 517 pp. Shrivastava, R. K. and Verma, K. K. 1982. Peritrophic membrane cells and their digestive function in Aspidomorpha miliaris (Coleoptera, Chrysomelidae). Indian Journal of Experimental Biology 20: 595-599. Verma, K. K. and Shrivastava, R. K. 1989. Differential distribution of enzyme activity in the mid-gut of Aspidomorpha miliaris (Coleoptera, Chrysomelidae). Entomography 6: 373-379. Wigglesworth, V B., 1953. The Principles of Insect Physiology (5th Edn.). Methuen and Co. Ltd., London.

duct of salivary gland \

oesophagus *

buccal chamber gastric or mesenteric

oesophagus *

rectum gizzard .' or salivary proventriculus gland

Malpighian tubules rectum mouth

mandible

- Fig. 21.1. Typical insect digestive system in lateral view (original).

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