APPENDIX. 399 



•circular eye-spots black (Fiji'. .'{. 6. A greenish area shows along the pos- 

 terior two-thirds of the body, which is merely the remains of the last meal 

 appearing through the transparent skin. The larva does not feed after this 

 moult, but crawls down the plant to the ground, \vbich it burrows into act- 

 ively, disappearing beneath the surface in a vei'y short time. It penetrates 

 to a depth of from half an inch to two or three inches, usually the lesser 

 •distance, and at the extremity of the burrow presses the soil away from 

 itself, so as to fomi a little cell or chamber, the sides of which it moistens 

 with saliva. The drying and hardening of the walls of the chamber form a 

 sort of cocoon of lirm texture and more or less impervious to water (see 

 Fig. 4, a). 



During the heated season of July and August the transformation from 

 the larval to the pupal stage and from the latter to the adult insect is quite 

 rapid, the pupal stage being assumed in from six to eight days, and the 

 a,dult flies transforming and digging out through the soil some twelve or 

 fifteen days after the larva entered it. 



It seems from the studies by Peck, and in part confirmed by my own 

 observations, that all of the larv?e of the spring brood do not transform at 

 once, but some few of them remain unchanged as contracted dormant larvae 

 over winter to transform the next spring. It is true also of the second 

 broods of larvi* that some of them come out the same season, while others 

 remain over winter and do not pupate until shortly before the appearance 

 of the adults in April and May. The holding over to the next year of cer- 

 tain larvie of ench brood is doubtless a provision of nature to prevent the 

 extermination of the species by any untoward accident, such as the absence 

 of food, unfavorable climatic conditions, or abundance of natural enemies 

 which might prove disastrous to the species should all appear at once. 



In the latitude of Washington, D. C, the first brood of larvge practically 

 all disappear from the trees by the end of June, and the first flies of the 

 second brood begin to appear about June 20, and are out in greatest number 

 about the first of July. It is the progeny of this second brood of flies that is 

 particularly disastrous to the trees, although the spring brood of larvae is 

 often sufficiently abundant to do very serious injury. 



NATURAL ENEMIES. 



The slimy repellant covering of the larva does not altogether prevent its 

 being preyed upon by parasitic insects, and in Europe some ^half^dozen 

 parasites have been reared from it. In this country Peck [mentions a 

 minute parasitic fly, determined by Westwood as a species of Encyrtus, 

 which stings the egg of the slug-fly through the upper epidermis of the leaf, 

 placing in each egg of its host a single one of its own — much more minute. 



The little parasitic maggot when it hatches finds food enough within the 

 €igg of the slug-fly for the needs of its full development, changes to the chrys- 

 alis therein, and ultimately emerges a perfect fly like its parent. So 

 abundant is this parasite at times, as reported by Peck, that the'second 

 litter of eggs is sometimes nearly all destroyed. I have found evidence of 

 the occurrence at Washington, D. C, of this or some related~parasite. but 

 failed to secure the adult insect. 



