called mycetocytcs, for the bacteria to live in. A vinegar fly, Drosophila melano- Life Processes gaster, has been reared under sterile conditions on a definite chemical me- of Insects dium. It is the first multicelled organ- ism to be raised in the absence of Frank H. Bah er s ^ John J. Pratt ^ Jr. micro-organisms on a diet whose chem- ical composition was exactly known. Certain insects, such as mealworms, A study of insect physiology can tell require little water because they can us a great deal about the phenomenon derive metabolic water from carbo- of life and help entomologists in their hydrates. fight against insect pests. The ability to utilize sugars varies Of especial interest is nutrition, for considerably. Mannose is used by blow the digestive systems of insects are as flies and vinegar flies but not by bees. diverse as the insects themselves and Only aphids are able to use arabinose. the kinds of food they eat. When all Vinegar flies can survive for long pe- the evidence on that complex subject is riods on a diet of pure sucrose, rafíi- in we will be that much closer to the nosc, or melezitosc. solution to some mysteries that still To grow, insects must have proteins confront the biologist, physiologist, and or their equivalent. Some mature in- biochemist. sects can survive a long time on a pro- Some species eat almost anything, tein-free diet, but they either undergo but some have a restricted diet. Some no further development in their adult Piave been given more than one com- stages or utilize food materials already mon name because they customarily stored in the body. Certain amino feed on more than one plant: Boll- acids, the building blocks of proteins, worm, tomato fruitworm, and corn seem to be essential for proper growth earworm, for instance, are one insect. and development; the German cock- The influence of diet on the growth roach requires at least iivc, valine, is illustrated by the honey bee. Larvae tryptophane, histidine, arginine, and that are to become queens are fed on cystine. a diet of royal jelly. Other larvae des- Ectoparasites such as lice seem to tined to become workers are fed on develop better on vitamin-deficient royal jelly for only 2 days and for the rabbits than on well-fed ones. When rest of their larval life receive honey human volunteers were fed for several and pollen. months on a diet deficient in certain Several factors or vitamins are vitamins and then infested with lice, necessary for normal growth. The fat- however, the lice developed just as well soluble factors so important in mam- as they did on humans that had a com- malian physiology, except for choles- plete diet. On the other hand, it seems terol, apparently are not required by true that various insects often develop a number of insects. The water-soluble better and in greater numbers on plants factors do play an important role. Most with nutritional deficiencies than on species need the B vitamins. Vitamin well-fed plants. Powder-post beetles C does not seem to be required, but at cannot digest cellulose. If allowed to least one insect, the cockroach, synthe- choose among pieces of oak sapwood sizes vitamin C. Symbionts are bacteria of diflercnt starch content, the female that are transmitted hereditarily from almost always chooses the wood with parent insect to offspring. Apparently the highest starch content in which to these bacteria are essential in the nu- lay her eggs. trition of many insects. Sometimes the Metabolism is the sum of all the insects provide specialized structures. chemical and physical processes by 30 Life Processes of Insects 31 which living organized substance is processes. The oxygen consumption of produced and maintained. The subject cockroach muscle is about the same as obviously is complex and in only a few that of pigeon-breast muscle, which instances has the metabolism of an in- heretofore has been considered the gested food been followed completely. most active tissue known. What happens to the blood pigment Besides the usual waste products of hemoglobin after it is ingested by metabolism, many insects excrete ma- bloodsucking arthropods has been in- terials like wax and silk, which they vestigated. In most of the insects stud- use for various purposes. Other sub- ied the bulk of hemoglobin seems to be stances, such as the fetid material ex- broken down in the gut to hcmatin, creted by stink bugs, are used for pro- which is then excreted unchanged. In tection. Still others, such as the venom mosquitoes and fleas, no pigment seems of the wasp, are used in obtaining food. to be absorbed. In all the other forms, Radioactive amino acids have been pigment in varying amounts is ab- injected into the giant silkworm and sorbed and circulates in the hemo- apparently radioactive silk was ob- lymph. In the louse the absorbed pig- tained. The studies will help explain ment is further broken down to the bile the chemical structure of silk. pigment, biliverdin, and in other The naturalist Athanasius Kircher species bilirubin is also found. in 1643 recommended music as an During metamorphosis, the period antidote for tarantula bites. Different during which the insect changes from treatments are used today for insect an immature stage to an adult, the de- bites, but often they are no more efTec- hydrogenase enzym.e activity in the tive than Kircher's. We know little blow fly falls rapidly at first, reaches a about the nature of insect venom. In minimum at about the halfway point some ants it is formic acid; in others, of the pupal period, then rises rapidly toxic protein. Bee venom is made up of and continuously until metamorphosis several toxic constituents, the chief of is completed. The acidity of the pupal which is apitoxin. When it is injected fluid follows a somewhat différent by the sting of the bee, enzymes in the course, becoming strongly acid soon toxin cause a breakdown of cell proto- after the beginning of metamorphosis, plasm and the liberation of histamine. and reaches a maximum, at about the It is this chemical that is responsible same time the dehydrogenase activity for many of the symptoms of bee sting. is lowest. The acidity then decreases Since early times bee venom often has until the time for emergence, when the been recommended for the treatment fluids are almost neutral. In the Japa- of arthritis, neuritis, and rheumatism. nese beetle, the changes in fat and gly- Another mystery is the nature of the cogen content during metamorphosis salivary gland secretion of various mos- may indicate that the insect synthesized quitoes and flies. A toxic arrow poison glycogen from fat. used by the Bushmen of the Kalahari The metabolism of iodine by vinegar Desert in South Africa is obtained from flics was studied by the use of radio- the larva of the beetle Diamphidia active iodine (P^^). When it was fed to locusta. larvae, the iodine was concentrated Insects do not have blood vessels. mostly in the protein of the skeletal The circulating fluid flows freely parts of the larvae. If the pupae formed throughout the body cavity except from larvae fed radioactive iodine were while it is being moved by the dorsal removed from the food before emer- vessel or heart. It corresponds to both gence, the adult insects did not contain blood and lymph and is called hemo- radioactive material. lymph. In some insects it is clear and The amount of oxygen consumed by colorless. In others it is yellow or green. tissues during metabolism is an indica- The volume varies greatly betv/een spe- tion of the activity of the metabolic cies and individuals of one species. The 32 Yearbook of Agriculture 1952 hemolymph does not contain respira- Hemolymph from Japanese beetle tory pigments such as hemoglobin or grubs coagulates by a gelation of the hemocyanin. Many analyses of hemo- plasma, while that from the wax moth lymph have been made, but the func- coagulates by agglutination of the cells. tion of only a few of the many compo- The coagulation of the hemolymph nents has been determined. from these two species may be greatly Insect hemolymph contains more retarded by exposing the larvae to sub- free amino acids than does human lethal intensities of ultrasonic waves. blood, which averages about 6 milli- None of the chemicals normally used grams per hundred milliliters. Insect to prevent the clotting of mammalian blood may contain as high as 385 mil- blood has a similar elïect on insect ligrams per hundred milliliters. At blood. least 24 compounds with the chemical properties of amino acids that occur Tw^ENTY or more species of insects free in the hemolymph of insects have have developed resistance to insecti- been identified by the use of paper cides following exposure to insecticides Chromatographie methods. Several of under natural conditions. Resistant them have not been identified yet as strains have been developed in the lab- constituents of proteins. oratory by exposing many insects to In most insects the hemolymph con- concentrations of insecticide that killed tains a much higher percentage of po- 90 percent of them. Eggs from the sur- tassium than docs mammalian blood. vivors were used to maintain a colony. Among phytophagous, or plant-feed- The process was repeated with each ing insects, the sodium-potassium ratio generation. In a short time the off- is less than i ; among carnivorous in- spring showed considerable tolerance sects, the ratio is greater than i. Some for the insecticide used in the selective species of insects apparently have some process and also, usually, for many sort of sodium-potassium regulatory chemically unrelated compounds.