INSECT COMMUNITIES 275 



silken thread to which the leaders of succeeding files 

 attach threads of their own making, thus bringing the 

 whole column into unison. Caterpillar colonies usually 

 break up when a certain stage of growth is reached, or 

 when the food supply in the neighbourhood of their nest 

 is exhausted ; but certain species are continuously gre- 

 garious, and eventually spin compound cocoons. 



Insects sometimes assemble in vast companies for the 

 purpose of travel, certain kinds of larvae having gained the 

 title of " army worms " on this account. For some un- 

 explained reason, the half-grown larvae of a " wainscot " 

 moth (Leucania unipunctata) suddenly become gregarious, 

 and migrate in vast hordes which overrun grass and corn 

 lands, where they work incalculable havoc. These in- 

 vasions take place in North America, but the species is 

 almost cosmopolitan, and its native country is unknown. 

 Other so-called " army worms " are the maggots of small 

 midges belonging to the genus Sciara. They live under 

 layers of rotting leaves in the forests of Northern Europe, 

 and at times migrate from one feeding ground to another 

 in weird, snake-like columns. Millions of the maggots, 

 held together by their viscous secretions, form great strings 

 or ribbons several inches thick and many feet in length. 

 At the end of their journey they collect into one writhing 

 mass, which is slowly dissipated as its units burrow beneath 

 the leaves In search of food. These processional maggots 

 are also found in the United States. 



Certain adult insects habitually congregate in flocks 

 after the manner of birds, and it is noteworthy that these 

 gatherings usually consist of males only. The writer has 

 seen thousands of chafer beetles flying about street lamps. 

 Some hundreds were caught and examined, but not a 

 single female was found. Clouds of male flies of the 

 genus Bibio may often be observed in rapid, whirling 



