106 AN ACCOUNT OF BRITISH FLIES. 
bilobed structure. Ocelli present, three in number, one much smaller 
than the other two. Eyes large, emarginate. No transverse sutures 
onthethorax. Haltereslong. Abdomen elongated, seven segments. . 
Legs slender, and slightly elongated ; tibize armed with two spurs. 
Their habitat is in grass, flowers, fruit and fungi.* The larve, 
which are cylindrical and shining white, live under the bark of de- | 
caying trees, also in fungi and Aodeti, 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 pupz have two divergent caudal sete, and 
remain under the bark of trees and in the fungi; a cocoon may 
Or may not be spun. In some respects the larvae resemble 
Mycetophila \arve, but they have no bristles or spines on the 
locomotive processes on the venter, as in the JZycetophile. One 
of the most remarkable characters of the larve of this family is their 
gregarious and migratory character. From the latter the name of 
‘Army Worm,” or in Germany “ Heerwurm,” has arisen. At certain 
times of the year large companies 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 4 inch thick. This dense layer of 
larvee 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. 
What this migration is for, still seems doubtful; the larve usually 
being full fed, it would not be for change of feeding ground. But 
Beling,{ 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 larvee full fed. It was usually believed that the larve 
forming these masses were those of S. Thome, but Beling tells us 
they are those of S. mé/itaris ; in one case only did the larvze 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 92. Mr. Dale has more than once seen them flying in that position. 
t Insecta Lapponica, p. 825. 
{ Der Heerwurm (Zeit. f. die. ges. Naturw. v. 46, 1883, pp. 253-271). 
§ Proc. Ac. Nat. Sc. Philad., 1867, p. 222. 
