ZYGOBOTHRIA XIDICOLA, PARASITE OF BROWX-TAIL MOTH. 7 



i may perhaps result from a direct secretion of the hymenopterous 

 species, or it may follow some special reaction on the part of the 

 host ; at no time has evidence of active combat been found. 



In the spring the brown-tail moth larvae that have hibernated begin 

 feeding as soon as the buds open, but the Zygobothria maggots, in 

 their cysts in the oesophagus, remain inactive for several weeks 

 longer. It is not until late May and early June, when the host larvae 

 have molted into the last stage, that the parasite leaves its cyst in 

 the fore-intestine and again enters the body cavity of the caterpillar 

 to feed. Invariably it works its way at once to the posterior end of 

 the host. After three or four days it is found to have established 

 communication with the outside air through an opening in the integu- 

 ment of the brown-tail moth larva. From this minute, more or less 

 circular opening there proceeds a rapid growth of the integument 

 into the body cavity. This ingrowth takes the form of a funnel, 

 within which the parasite lies, its posterior end directed toward the 

 small opening, its anterior end free in the fat and fluids of its host. 

 Thus the parasite has procured for itself an independent air supply. 

 On the outside of the integumental funnel layers of soft tissue, 

 evidently consisting of hypodermal cells, leucocytes, and compressed 

 fat -cells of the caterpillar, are gradually laid down, one upon 

 another, until a thick, flesh}' wall has been formed about the funnel. 



The manner in which the opening through the body wall of the 

 brown-tail moth larva is effected was not observed. Possibly it results 

 from irritation by the spines at the caudal end of the parasitic 

 maggot. 



SECOND-STAGE MAGGOT OF THE PARASITE. 



Very soon after becoming established in the posterior end of its 

 host, and in the integumental funnel, the parasite molts into the 

 second stage. The first-stage skin is pushed back upon the funnel 

 where it is readily detected by the presence of the mouth hook. The 

 second-stage maggot is distinguished from that of the first stage by 

 its larger size, the much heavier mouth hook, and the presence of a 

 pair of anterior spiracles, often difficult to locate, situated between 

 the second and third body segments. Instead of the single apical 

 tooth of the first stage the mouth hook now has two teeth, the 

 pharyngeal skeleton having divided longitudinally over its anterior 

 half; furthermore, there is now an indistinct transverse joint near 

 the middle of this anterior portion. The posterior plates of the 

 mouth hook are much stouter than in the first stage. 



Throughout this instar, which requires from 8 to 12 days, the 

 maggot remains in the integumental funnel. It grows rapidly during 

 this period so that it measures about 4 mm. in length when ready to 

 molt into the third and last larval stage. By this time the host has 

 spun its cocoon. 



