﻿346 J. DEWITZ — THE BEARING OF PHYSIOLOGY 



gardens for collecting below them various invertebrates such as earwigs, 

 slugs, etc., and then destroying them wholesale. Both P. Marchal 27 and I 11 have 

 arrived at the conclusion that the female of the grape-moth, Clysia ambiguella, 

 when laying its eggs on the flower-buds and berries of the vine, is guided by the 

 highly sensitive extremity of its abdomen. And this sensitiveness of the point 

 of the abdomen also causes the females of many insects to lay their eggs in 

 fissures and folds of plants, of the soil, and other substances. One can easily 

 watch such females feeling their way searchingly with the finger-shaped out- 

 stretched abdomen. According to Seitz 33 full-grown caterpillars of Gastropacha 

 neustria, which at that time live solitary, adhere lengthways to one another in 

 two and threes, thus incontestibly demonstrating their reaction to the stimulus of 

 contact. This observation may lead us to the causes of the gregarious habits of 

 many insects. It is well known that many caterpillars, when young, live huddled 

 together in common nests, thereby facilitating their destruction. But later on, 

 under the influence of physiological changes in their internal conditions, these 

 insects separate and lead a solitary, individualistic life. Under certain influences 

 their former gregarious and socialistic mode of existence can, however, be 

 restored. Such conditions are also found amongst higher animals. Birds 

 assemble and form enormous swarms at certain seasons, fishes congregate and in 

 their millions ascend rivers or descend into the sea when their products of 

 reproduction are ripening. Hence age, season, or reproduction may evidently 

 influence individual animals to become gregarious. According to J. Loeb the 

 olfactory sense also performs an important function in this connection. 



Geotropism, or the tendency of living organisms to be attracted towards the 

 centre of the earth, may frequently combine with phototropism and thereby 

 force animals to locate themselves on the extreme ends of tree branches and on 

 the crowns of trees (negative geotropism), or to descend into the soil ( positive 

 geotropism). 



Rheotropism, into which I 5 have made researches in connection with various 

 classes of lower animals and which is particularly pronounced in those that live 

 in water, shows itself in the tendency and efforts of animals to head against the 

 current of water or air and to remain or move in that position. This fact can 

 readily be observed with fishes in small streams. Or one may fill a large round 

 glass jar with water, then place in it some water animals — say newts — and then 

 move the water round. The newts in the water or the Hydrometra, which race 

 about on the surface, will head at once against the current. So far as the air 

 currents themselves are concerned, the study of rheotropism has received scant 

 attention. Wheeler 30 calls this anemotropism and defines it as signifying the 

 tendency and efforts of those insects which float and glide about in the air to face 

 the wind current. To such insects belong Bibio, Anthomyia, Syrphus, Bombylius, 

 Sphingidae, and Odoxata. Amongst flies sometimes only males are anemo- 

 tropic ; Osten-Sacken found that with the Diptera the faculty of floating in the 

 air is closely connected with the holoptic heads of the males. The Rocky 

 Mountain locusts (Melanoplus spretus) fly with the wind in a light breeze, but as 

 soon as the velocity of the wind rises they fly against it. 



It is not at all unlikely that rheotropism causes many species to build their 

 nests and habitations facing the prevailing winds. Schmarda 3 mentions that in 



