ZYGOBOTHRIA NIDICOLA, PARASITE OF BROWX-TAIL MOTH. 5 



the caterpillars, she slowly and deliberately pushed her abdomen 

 downward and forward until the ovipositor plates were even with 

 her face. With a quick movement the ovipositor was then pushed 

 beneath the larva, and an egg with a first-stage maggot within it 

 was deposited. The egg is almost invariably placed on the venter 

 of the host, and usually occupies a transverse position between two 

 pairs of true legs, or, less frequently, between two pairs of prolegs. 

 Occasionally an egg is placed on the dorsum by accident, but m such 

 cases the parasitic maggot is unable to enter its host — at least this 

 was true of instances under observation in the laboratory. The ex- 

 planation probably is to be found in the thicker skin of the dorsum, 

 which is not so easily pierced by the small maggot. 



Although only one parasite can complete its development in one 

 host larva, the fly uses no discrimination when depositing her eggs ; 

 she places eggs as readily upon larvae already having eggs upon 

 them as upon those not yet attacked. From five to eight eggs have 

 been found on one caterpillar. That this takes place under field con- 

 ditions as well as in the laboratory has been disclosed by dissections ; 

 from 6 to 10 first-stage maggots of the parasite have been not un- 

 commonly dissected from single field-collected brown-tail moth 

 caterpillars. 



EGG. 



As deposited, the egg measures from 0.42 to 0.45 mm. in length by 

 0.11 to 0.12 mm. in width; the maggot within is 0.33 to 0.35 mm. 

 long and 0.08 to 0.09 mm. broad. In form the egg is elongate-oval, 

 somewhat narrowed at the posterior end, and concave on the lower 

 side: in color it is whitish. The thin and delicate chorion is trans- 

 parent. When viewed from above the egg appears opaque; this is 

 due to what seems to be a special layer of protecting tissue just in- 

 side the chorion; it is limited to the posterior three-fourths of the 

 egg, and occurs only above the maggot. It is peculiarly reticulated, 

 being marked off into very slender hexagons, the outermost of which 

 are incomplete. Its position suggests its function to be that of afford- 

 ing protection to the young maggot before the latter succeeds in bor- 

 ing into its host. 



ENTRANCE OF MAGGOT INTO HOST. 



Having been placed upon its host the parasitic maggot begins to 

 cut through the thin egg chorion that confines it, and as soon as this 

 is clone it bores into the caterpillar. The posterior end of the para- 

 site remains inside the eggshell until the opening into the host has 

 been made; when this has been accomplished it requires but a 

 fraction of a second for the maggot to pull its whole body into the 

 caterpillar. In one case under observation the entire process of 

 cutting through the egg chorion and the host skin and entering the 

 104616—22 2 



