144 HUMAN BIOLOGY 



difTerent individuals. All animals give oft heat, moisture, 

 carbon dioxide, secretions and excretions and make move- 

 ments. Simple tropistic or reflex responses to these, such 

 as those designated by the terms thermotropism, hygro- 

 tropism, chemotropism and stereotropism, are probably 

 sufficient to account for many aggregations and associations. 

 Migratory crickets have been observed to huddle together 

 for mutual warmth when the cool of evening comes on; 

 slaters (Oniscus) are induced to assemble by the moisture 

 which they give off", and resting locusts may be stimulated 

 to flight by the movements of their feflows. Rhythmic 

 emission of hght in fireflies or of chirping in crickets may 

 excite rhythmic, and according to some authors synchronous, 

 responses of the same kind in other individuals in the imme- 

 diate neighborhood ("physiological sympathy" of Ribot). 

 Some simple aggregations are evidently the result of a 

 number of tropistic responses. One example will suffice. 



The larvae of the common blackfly (SimuKum) often 

 congregate in dense masses on stones in the more torrential 

 parts of our streams, stand erect on their posterior ends and 

 capture with their out-spread, rake-hke mouthparts diatoms 

 and other microorganisms as they float past. In this case 

 we may distinguish stereotropic responses of the larvae to 

 sohd bodies (the stones), rheotropic responses to the current 

 and probably also chemotropic responses to the higher 

 oxygen content of the more rapidly moving water. It wifl 

 be noticed that this combination of tropistic responses 

 constitutes an exquisite adaptation because it places the 

 stationary larvae in the optimal enviroment for securing their 

 food, since much more of it passes within their reach in a given 

 time in the torrential than in the sluggish portions of the 

 stream. Although aggregation here actually brings about a 

 competition for the food among the individuals, this dis- 

 advantage is more than compensated by the increased 

 supply due to the swiftness of the stream. There is an exten- 

 sive and interesting literature, much of which has been 

 reviewed by Allee, dealing with the effects of aggregations 

 on their component individuals, but the subject cannot be 

 further elaborated in this article. 



