1938: Entomology

Important developments have marked the year 1938 in all fields of entomological research, ranging from the practical aspects of medical and economic entomology to those of the more general fields of embryology, genetics, and physiology.

Disease-bearing Insects.

After the demonstration by Walter Reed and his associates that yellow fever is carried by the Aedes aegypti, it was thought that eradication of the disease through mosquito-control measures was possible. While this method has been successful in the United States, other countries, such as Africa and South America, have continued to suffer from outbreaks of this disease. It has been shown that the chief reason for these continued outbreaks is that the yellow fever virus can be carried by insects other than the aegypti mosquito. This has been recently demonstrated by a group of Rockefeller scientists working under the Cooperative Yellow Fever Service of Brazil and the International Health Board. And scientists at Harvard and Cornell Universities have shown that one of our own widely distributed species of mosquitoes (Aedes triseriatus) is also capable of carrying the yellow-fever virus. However, there appears to be no immediate danger from this source, because the ability to carry the virus has only been demonstrated in the laboratory, and this species of mosquito is probably not a natural carrier of the disease.

Another front in mankind's war against insects also seems to give some promise of victory — that in Africa against the dreaded tsetse fly. This insect is a bearer of the deadly African sleeping sickness, and a scourge to humans as well as to livestock. The discovery of the high value of a larvicide as a tsetse-fly repellent was made by a recent American expedition to Africa. The larvicide was originally developed for the control of mosquitoes in New Jersey by Dr. J. M. Ginsburg, a biochemist from Rutgers University.

A serious insect pest in the vineyards has been found to be susceptible to the fatal lure of a pale blue light. This work was reported by J. K. Ellsworth of the University of California. The interesting fact brought out in this series of experiments is that the female grape leafhopper seems to respond more readily to the blue light than do the males. While the factors responsible for this sexual differentiation to light have not been explained, the results can still be termed a success from the economic viewpoint, in that it is the females which in turn produce the new crop of leafhoppers.

The deer botfly's speed has been widely reported as being approximately 800 miles per hour. This now seems to have been completely repudiated by the experiments of an engineer. Dr. Irving Langmuir, of the General Electric Company's research laboratories. Dr. Langmuir proved that this speed would be an impossibility on the basis of his theoretical calculations and laboratory experiments. He states that if such a high speed were attained by this insect, it would consume one and one half times its own weight in food every second in flight, which would be impossible. Furthermore, if the insect struck human flesh at 800 miles an hour, it would exert a force of 310 lb. and be a fairly efficient bullet, causing a serious wound. Such wounds have never been reported. His experiments also showed that, at a speed of 64 miles per hour, the deer botfly would be invisible. Hence. Dr. Langmuir concludes that 25 miles per hour would be a more reasonable speed for this species of insect. Also, in regard to the speed of insects in flight, a well-known New Zealand entomologist, Dr. R. J. Tillyard, observed the speed of a dragon fly over a measured course and found its speed to be 60 miles per hour. Other scientists report speeds of flying insects of various species to be between 18 and 33 miles per hour.

Experiments on Protective Coloration and Food Specificity.

Despite recent claims to the contrary, Dr. Isley of Trinity University, Waxahachie, Tex., has shown that protective coloration of insects really does protect them against their more common enemies, the birds. Dr. isley's experiments showed that 88 per cent of the various insects that contrasted with their backgrounds were found and eaten by the birds, whereas only 40 per cent of the insects were eaten by the birds when the backgrounds harmonized with the insect's own color.

Dr. Isley has also carried out another series of interesting ecological studies. He made observations on the food of some forty species of grasshoppers and found that 50 per cent were primarily grass feeders. The other 50 per cent chose their host plants from the forbs or broad-leaf flowering plants. The interesting phase of this series of observations seems to be that this food specificity is not only a physiological fact but also a morphological adaptation. Microscopic and macroscopic examination of the mandibles of these grasshoppers showed two chief patterns, and these two patterns conformed to the two general types of feeding habits of these forty species of grasshoppers.

Sense Organ Experiments — Hearing.

Interesting experiments in the study of special sense organs of insects has been continued this year. One study was made by Dr. F. L. Wells, of the Harvard Medical School. He has shown that spiders can hear at least some of the sounds audible to the human car. He tested a number of species of orb-weaving spider with the use of a tuning fork of medium pitch, holding the instrument close to them but not touching them. The spiders showed various consistent responses, ranging from slight leg movements to actual attacks upon the fork, as if it were an insect.

Studies on Insect Development.

Studies on the chromosomes of the salivary gland cells of certain insects is being continued by the school of scientists at the Carnegie Institution of Washington in an effort to more firmly establish the structural units of these small strands of nuclear material, which are so important as bearers of hereditary characteristics in all forms of life. Also in connection with these studies in cytology other studies on the determination of structural units and factors which influence developmental changes during the life of the insect are being carried out in a number of research institutions. The results of these studies on insect development serve as additional stimulation toward speculation concerning the problem of development in higher animals, and also to emphasize the importance which comparative embryology would have for a better understanding of these processes.

Cellular Experiments.

In the field of cellular physiology, the use of insects in the laboratory for determining the properties and potentialities of such substances as activators or enzymes, is being carried on more and more extensively each year.