Life Processes of Insects

Frank H. Babers, John J. Pratt, Jr.

A study of insect physiology can tell us a great deal about the phenomenon of life and help entomologists in their fight against insect pests.

Of especial interest is nutrition, far the digestive systems of insects are as diverse as the insects themselves and the kinds of food they eat. When all the evidence on that complex subject is in we will be that much closer to the solution to some mysteries that still confront the biologist, physiologist, and biochemist.

Some species eat almost anything, but some have a restricted diet. Some have been given more than one common name because they customarily feed on more than one plant : Bollworm, tomato fruitworm, and corn earworm, for instance, are one insect.

The influence of diet on the growth is illustrated by the honey bee. Larvae that are to become queens are fed on a diet of royal jelly. Other larvae destined to become workers are fed on royal jelly for only 2 days and for the rest of their larval life receive honey and pollen.

Several factors or vitamins are necessary for normal growth. The fat-soluble factors so important in mammalian physiology, except for cholesterol, apparently are not required by a number of insects. The water-soluble factors do play an important role. Most species need the B vitamins. Vitamin C does not seem to be required, but at least one insect, the cockroach, synthesizes vitamin C. Symbionts are bacteria that are transmitted hereditarily from parent insect to offspring. Apparently these bacteria are essential in the nutrition of many insects. Sometimes the insects provide specialized structures,

called mycetocytes, for the bacteria to live in.

A vinegar fly, Drosophila melanogaster, has been reared under sterile conditions on a definite chemical medium. It is the first multicelled organism to be raised in the absence of micro-organisms on a diet whose chemical composition was exactly known.

Certain insects, such as mealworms, require little water because they can derive metabolic water from carbohydrates.

The ability to utilize sugars varies considerably. Mannose is used by blow flies and vinegar flies but not by bees. Only aphids are able to use arabinose. Vinegar flies can survive for long periods on a diet of pure sucrose, raffinose, or melezitose.

To grow, insects must have proteins or their equivalent. Some mature insects can survive a long time on a protein-free diet, but they either undergo no further development in their adult stages or utilize food materials already stored in the body. Certain amino acids, the building blocks of proteins, seem to be essential for proper growth and development; the German cockroach requires at least five, valine, tryptophane, histidine, arginine, and cystine.

Ectoparasites such as lice seem to develop better on vitamin-deficient rabbits than on well-fed ones. When human volunteers were fed for several months on a diet deficient in certain vitamins and then infested with lice, however, the lice developed just as well as they did on humans that had a complete diet. On the other hand, it seems true that various insects often develop better and in greater numbers on plants with nutritional deficiencies than on well-fed plants. Powder-post beetles cannot digest cellulose. If allowed to choose among pieces of oak sapwood of different starch content, the female almost always chooses the wood with the highest starch content in which to lay her eggs.

Metabolism is the sum of all the chemical and physical processes by which living organized substance is produced and maintained. The subject obviously is complex and in only a few instances has the metabolism of an ingested food been followed completely.

What happens to the blood pigment hemoglobin after it is ingested by bloodsucking arthropods has been investigated. In most of the insects studied the bulk of hemoglobin seems to be broken down in the gut to hematin, which is then excreted unchanged. In mosquitoes and fleas, no pigment seems to be absorbed. In all the other forms, pigment in varying amounts is absorbed and circulates in the hemolymph. In the louse the absorbed pigment is further broken down to the bile pigment, biliverdin, and in other species bilirubin is also found.

During metamorphosis, the period during which the insect changes from an immature stage to an adult, the dehydrogenase enzyme activity in the blow fly falls rapidly at first, reaches a minimum at about the halfway point of the pupal period, then rises rapidly and continuously until metamorphosis is completed. The acidity of the pupal fluid follows a somewhat different course, becoming strongly acid soon after the beginning of metamorphosis, and reaches a maximum at about the same time the dehydrogenase activity is lowest. The acidity then decreases until the time for emergence, when the fluids are almost neutral. In the Japanese beetle, the changes in fat and glycogen content during metamorphosis may indicate that the insect synthesized glycogen from fat.

The metabolism of iodine by vinegar flies was studied by the use of radioactive iodine (I¹²¹). When it was fed to larvae, the iodine was concentrated mostly in the protein of the skeletal parts of the larvae. If the pupae formed from larvae fed radioactive iodine were removed from the food before emergence, the adult insects did not contain radioactive material.

The amount of oxygen consumed by tissues during metabolism is an indication of the activity of the metabolic processes. The oxygen consumption of cockroach muscle is about the same as that of pigeon-breast muscle, which heretofore has been considered the most active tissue known.

Besides the usual waste products of metabolism, many insects excrete materials like wax and silk, which they use for various purposes. Other substances, such as the fetid material excreted by stink bugs, are used for protection. Still others, such as the venom of the wasp, are used in obtaining food.

Radioactive amino acids have been injected into the giant silkworm and apparently radioactive silk was obtained. The studies will help explain the chemical structure of silk.

The naturalist Athanasius Kircher in 1643 recommended music as an antidote for tarantula bites. Different treatments are used today for insect bites, but often they are no more effective than Kircher's. We know little about the nature of insect venom. In some ants it is formic acid; in others, toxic protein. Bee venom is made up of several toxic constituents, the chief of which is apitoxin. When it is injected by the sting of the bee, enzymes in the toxin cause a breakdown of cell protoplasm and the liberation of histamine. It is this chemical that is responsible for many of the symptoms of bee sting. Since early times bee venom often has been recommended for the treatment of arthritis, neuritis, and rheumatism.

Another mystery is the nature of the salivary gland secretion of various mosquitoes and flies. A toxic arrow poison used by the Bushmen of the Kalahari Desert in South Africa is obtained from the larva of the beetle Diamphidia locusta.

Insects do not have blood vessels. The circulating fluid flows freely throughout the body cavity except while it is being moved by the dorsal vessel or heart. It corresponds to both blood and lymph and is called hemolymph. In some insects it is clear and colorless. In others it is yellow or green. The volume varies greatly between species and individuals of one species. The hemolymph does not contain respiratory pigments such as hemoglobin or hemocyanin. Many analyses of hemolymph have been made, but the function of only a few of the many components has been determined.

Insect hemolymph contains more free amino acids than does human blood, which averages about 6 milligrams per hundred milliliters. Insect blood may contain as high as 385 milligrams per hundred milliliters. At least 24 compounds with the chemical properties of amino acids that occur free in the hemolymph of insects have been identified by the use of paper chromatographic methods. Several of them have not been identified, yet as constituents of proteins.

In most insects the hemolymph contains a much higher percentage of potassium than does mammalian blood. Among phytophagous, or plant-feeding insects, the sodium-potassium ratio is less than 1; among carnivorous insects, the ratio is greater than 1. Some species of insects apparently have some sort of sodium-potassium regulatory system, because the ratio in the body fluid is not dependent on the ratio in food. In the silkworm larva, the sodium concentration in the body fluid seems to be in simple diffusion equilibrium with ingested sodium. Silkworm pupae and adults contain almost no sodium. It therefore must be selectively excreted.

Insect hemolymph contains a number of cells, or hemocytes. Their most obvious activity corresponds to that of the leucocytes, or white blood corpuscles, of the vertebrates in that they ingest any small particles of solid matter set free in the blood. Ten classes and 32 types of cells have been found in the blood of the southern army-worm, and 8 classes and 23 types of cells in the blood of the mealworm.

When removed from the insect, the hemolymph of some species clots rapidly and in others more slowly or not at all. The process of coagulation is not comparable to that of mammalian blood and varies between insect species.

Hemolymph from Japanese beetle grubs coagulates by a gelation of the plasma, while that from the wax moth coagulates by agglutination of the cells. The coagulation of the hemolymph from these two species may be greatly retarded by exposing the larvae to sub-lethal intensities of ultrasonic waves. None of the chemicals normally used to prevent the clotting of mammalian blood has a similar effect on insect blood.

TWENTY or more species of insects have developed resistance to insecticides following exposure to insecticides under natural conditions. Resistant strains have been developed in the laboratory by exposing many insects to concentrations of insecticide that killed go percent of them. Eggs from the survivors were used to maintain a colony. The process was repeated with each generation. In a short time the offspring showed considerable tolerance for the insecticide used in the selective process and also, usually, for many chemically unrelated compounds.

The control of the wild resistant insects has become a serious problem DDT, after a few years of use, often has failed to control house flies and mosquitoes. Apparently no external differences exist between susceptible and insecticide-resistant flies. Scientists have tried to find out whether there are physiological differences. They have yet found no significant difference in vigor between susceptible and resistant strains: Resistance is not due simply to the failure of the insecticide to penetrate the cuticle of the insect, because the insects are also resistant when the insecticide is injected directly into the body cavity.