The cuticle may be rigid, flexible, or elastic. It also is waterproof ; it has to be, because the integument keeps a proper water balance in the insect. The epicuticle, the thin outer membrane that is the most important in waterproofing the cuticle, is a complex structure of several layers. The first to be deposited is the innermost or cuticulin layer, believed to consist of a lipoprotein, which perhaps is denatured, condensed, and finally tanned along with other proteins present in the outer layers. Then a thick viscous fluid is discharged, and on top of that a wax layer. The wax layer is then topped with a hard cement layer, which is thought to consist of tanned proteins combined with lipids. The cement layer is secreted by some of the dermal glands whose openings are scattered over the surface of the integument.

Throughout the cuticle, running vertically from the cells, are the pore canals, of unknown function. In the cuticle of flesh fly larvae (Sarcophagidae), 15,000 of these were found per square millimeter.

The cuticle cannot grow and, in the rigid parts of the insect, cannot be stretched. As the insect grows, therefore, the cuticle is shed and is replaced by a larger cuticle. That process, molting, follows a period of great cell activity. When first laid down, the new cuticle is soft and often colorless, but it rapidly hardens and assumes its normal color.

The blow fly has been a favorite experimental insect because its larval cuticle is not shed before pupation, but rather is converted into the hard puparium. In the flesh fly (Sarcophagi barbata) the formation of the hard puparium from the soft larval cuticle is by the following process: Phenols are oxidized enzymatically by polyphenol oxidise to orthoquinone. The orthoquinone combines with the protein present and hardens it by a tanning process, during which the integument, which may have been colorless for a short period after the molt, becomes colored brown or black. The colors due to tanning, however, are not the basis for the brilliant iridescent or metallic colors of some insects. Such colors are due usually to the interference in the reflection of light from the multiple thin plates or scales that some insects have.

The shell, or chorion, of an insect's egg is like cuticle in many ways, but is even more complex. The shell of the egg of the assassin bug, Rhodnius prolixus, consists of seven layers, none of them waterproof. A cement layer is added to the outside of the egg when it is laid. Waterproofing is effected by a thin wax layer on the inside of the chorion, similar to the one that waterproofs the cuticle of most adult forms. The wax is secreted by the maturing egg and is attached securely to the innermost layer of the chorion. The other layers are modifications of various protein-like materials somewhat like those in the cuticle.

The development of high-speed cameras, with which many exposures per second are possible, and cathode-ray oscilloscopes, by which small changes in electrical potential can be accurately recorded, has aided the study of the physiology of insect flight. When certain insects are held so that their feet are in contact with a movable platform, the insect will rest quietly. If the platform is removed, the insect moves its wings as in flight and many experiments can be done while the insect is actually suspended in air under simulated flight conditions. If small electrodes are inserted among the flight muscles, potential changes can be measured and correlated with wing movement. Some butterflies move their wings at a leisurely 5 beats a second, but certain midges attain about 1,000 wing beats a second. The vinegar fly is capable of flights lasting up to 2 hours. At the start, wing beats are about 300 per second but at the end, when fatigue becomes evident, they are about 100 a second. Among the insects with slow frequencies of wing beats, the wing movements are completely synchronous with nerve impulses, but when the frequency of wing beat increases there is no synchrony.

Insects, like the vertebrates, have highly developed, specialized sensory receptors that can be stimulated by chemicals. The chemical senses of insects may be roughly classified as taste, smell, and the common chemical sense of vertebrates whereby response is made to such irritants as ammonia and chlorine. The structure of the organs of taste and smell of insects differ greatly from that of the vertebrates, but a striking similarity exists in the physiological behavior toward many compounds and in the way in which stimulation is brought about. As among the vertebrates, however, the distinction between taste and smell is based on unsatisfactory evidence. We cannot yet relegate either taste or smell in insects to specific areas of the body; areas of contact chemoreceptors have now been found on the mouth parts, tarsal leg segments, antennae, and ovipositors of various species, although the actual organs are not always known.

In seeking materials that will attract and repel insects, research workers have investigated the mechanical response of insects to thousands of compounds that vaporize at body temperatures. Many of the compounds are synthetic; many are natural materials of unknown composition.

One such is a substance secreted by the female gypsy moth. It will attract male gypsy moths over long distances.

The method by which the worker honey bees inform other bees of the location of a new food supply has been described by Karl von Frisch, of the University of Munich. It has long been known that worker bees returning to the hive often performed. a kind of dance on the comb, but the reason for the dance was obscure. Von Frisch found that by the direction and duration of their movements, during the dance, the worker bees transmitted to other workers the direction and distance from the hive to the new-found food. He observed the antics in the darkness of the hive by the use of red light, to which the bees are insensitive. He found that he could predict the distance to about 100 yards. Direction was accurate to about 3 . The system worked for any distance up to about 3.7 miles. For direction, the bees use the sun as an orienting point. They also are apparently sensitive to polarized light, which they can use to get their bearings, because they can fly accurately whether or not the sun is visible.

Dr. von Frisch's discoveries, like others we have discussed, throw new light into the mysteries of nature. More such discoveries will come. They will give us a better understanding of insect physiology, of better controls of insects, and, indeed, of all life processes, including our own.

FRANK H. BABERS, a biochemist, is in charge of a project that deals with the mode of action of insecticides and physiology of insects. He is a graduate of the University of Florida and Princeton University. He joined the Department of Agriculture in 1936. From 1946 to 1948 he was in charge of the chemical section of the Orlando, Fla., laboratory of the Bureau of Entomology and Plant Quarantine.

JOHN J. PRATT, JR., joined the research staff of the Bureau of Entomology and Plant Quarantine upon receiving his doctor's degree from Cornell University in 1948. His work concerns the study of the mode of action of insecticides, the development of resistance to insecticides by insects, and insect physiology.