1 o6 AN A ceo UNT OF BRITISH FLIES. 



bilobed structure. Ocelli present, three in number, one much smaller 

 than the other two. Eyes large, emarginate. No transverse sutures 

 on the thorax. Halteres long. Abdomen elongated, seven segments. 

 Legs slender, and slightly elongated ; tibiae armed with two spurs. 



Their habitat is in grass, flowers, fruit and fungi.* The larvae, 

 which are cylindrical and shining white, live under the bark of de- 

 caying trees, also in fungi and Boleti, and, according to Walker, in 

 putrescent galls. 



Zetterstedtt says " sub-cortice arborum hybernant," but this does 

 not always appear to be the case, as they sometimes hibernate in 

 hay and straw. The pupjB have two divergent caudal set^e, and 

 remain under the bark of trees and in the fungi ; a cocoon may 

 or may not be spun. In some respects the larvte resemble 

 Mycetophila larvae, but they have no bristles or spines on the 

 locomotive processes on the venter, as in the Mycetophikc. One 

 ot the most remarkable characters of the larvre of this family is their 

 gregarious and migratory character. From the latter the name of 

 "Army Worm," or in (rcrmany " Heerwurm," has arisen. At certain 

 times of the year large comi)anies of this Heerwurm migrate in a 

 long procession, sometimes as much as 14 feet in length, and 2 

 or 3 inches wide, and may be \ inch thick. This dense layer of 

 larvre must contain some millions of individuals, sticking close 

 together and crawling over and over one another. The processions of 

 the "Army Worm " are seen in Germany, Sweden, Russia and America. 

 ^^"hat this migration is for, still seems doubtful ; the larvae usually 

 being full fed, it would not be for change of feeding ground. But 

 Beling,J in a recent paper, comes to the conclusion that this is the 

 object of the march. This latter view seems most natural were 

 not the larvae full fed. It was usually believed that the larvae 

 forming these masses were those of S. ThomcB, but Beling tells us 

 they are those of S. militaris : in one case only did the larvce belong 

 to a different species. 



The " Army Worm " has also been made known in America by 

 Cope,§ who has published his observations on the appearance of this 

 worm in Pennsylvania ; and from this author's paper the following 

 notes are taken : The rate of advance about four inches in five 

 minutes, the hinder ones working their way over the top of the rest, 

 the whole mass thus taking up in the rear and laying down in the 



* A singular habit in this genus is that of two $ 's frequently copulating with the 

 same 9 • Mr. Dale has more than once seen them flying in that position. 

 t Iiisecta Lappoiu'ca, p. 825, 



X Der Heerwurm (Zeit. f. die. ges. Naturw. v. 46, 1883, pp. 253-271). 

 S Troc. Ac. Nat. .'^c. Philad., 1867, p. 222. 



