196 
Journal of Agricultural Research 
Vol. XIV, No. s 
brown-tail-moth caterpillar, the Apanteles larva is minute and transparent, 
slightly less than 0.5 mm. in length, and possessing at its caudal end a 
curious bladder-like organ, commonly referred to as the anal vesicle, and 
beneath this a prominent fleshy horn projecting downward. On the 
dorsum of each of the last nine segments of the body is a transverse row 
of very indistinct, short, though rather stout, backward-projecting spines. 
When first removed from the caterpillar, the position usually taken by 
the parasitic larva is that shown in Plate 20, C, the body being curved 
so that the caudal horn touches or passes across the head. In this stage 
no tracheal system is visible. 
ANAL VESICLE OE APANTELES LACTEICOLOR 
The parasitic larvae of the subfamily to which this species belongs, the 
Microgasterinae, have at the caudal end of the body a bladder-like 
structure, called the “anal vesicle,” which has been the subject of much 
discussion, particularly in Europe, by such entomologists as Kulagin, 
Seurat, and Weissenberg. Weissenberg (S t 9), working principally on 
Apanteles glomeratus Linnaeus, formed a number of conclusions on the 
structure and function of the organ and summarized very well all the 
work that had been done upon this subject, besides giving the results of 
his own investigations. It has been determined definitely that the anal 
vesicle of these larvae consists merely of a portion of the hind gut which 
has been evaginated; all but the extreme posterior part of this section of 
the intestinal tract is concerned, being turned inside out so that the blind 
end of the midintestine is on the outer ventral surface of the vesicle. 
The vesicle is present in all endoparasitic stages of Apanteles spp., but is 
reinvaginated very shortly before the larva issues to spin its cocoon. Its 
function is not so definitely known, but there seem to be two important 
uses. The fact that the tracheal system of the parasite is not developed 
until the last endoparasitic stage, when the vesicle begins to be retracted, 
strongly suggests respiration as an important function, and the delicate 
structure of the organ would emphasize this. Frequently, when dis¬ 
secting brown-tail-moth caterpillars during the winter, the writer has 
found hibernating first-stage larvae of A. lacteicolor in which the hind 
intestine was not at all evaginated but could be distinctly seen within the 
larva (PI. 20, C). On several occasions while such a larva, placed in a 
drop of water, was under observation, the hind intestine was seen to be 
slowly pushed out through the anal opening to form the vesicle. While 
inactive within the host and under a comparatively low temperature, the 
respiration of the parasite is reduced to the minimum, and the slight 
respiration that does take place may go on through the body wall. When 
removed from the caterpillar and placed in water, the parasite becomes 
active and respiration increases. That at this time the vesicle should 
be formed supports the theory that an important function of the anal 
vesicle is respiration. However, Weissenberg (9) thinks, as a result of 
