July 7,1923 
Platygaster vernalis , Hessian Fly Parasite 
33 
THE PRIMARY LARVA 
In general outline the primary larva (PI. 2, A) is elongate oval, with 
length of body about three times its width and with a very slight taper 
toward the caudal end. Both extremities are bluntly rounded, and 
before the larva has become very much inflated with food a broad, deep 
constriction is evident on each side slightly posterior to the mouth. 
This constricted appearance is due to the greatly enlarged and projecting 
bases of the mandibles. 
The mandible (PI. 2, B) is noticeably long, being a little over one-third 
the width of the head. Measurements of single mandibles from four 
larvae show an average length of 0.073 millimeter. The mandible is 
wide at the base but tapers to a sharp extremity, with the distal third 
slightly curved. Several long, closely adhering spines are discernible 
along the curved portion and at the extremity. The entire mandible is 
nearly colorless and comparatively fragile. The mouth consists of a 
small transverse aperture, capable of being opened and closed by motion 
of the superior lip. 
The larva is usually sufficiently transparent to disclose under proper 
magnification certain outstanding features of the internal anatomy, such 
as the cells of the stomach wall and epithelium. Some of the cellular 
structures of the internal anatomy, including the stomach wall and 
proctodaeum, are illustrated in Plate 2, A. The details of cell structure 
were taken from microtomic sections. 
When the larva has developed sufficiently to feed, a movement of the 
labrum begins, thus producing a suction whereby the surrounding liquids 
or adjacent tender tissues are ingested. After freeing itself from the 
surrounding embryonic mass the larva first imbibes the green chyle from 
the host’s stomach and soon ingests particles of the stomach wall itself. 
While it may possibly secrete juices which have a softening effect on 
surrounding tissues, nevertheless undissolved particles of host tissue 
within the stomach of the parasite have been clearly seen, and in one 
case part of the stomach tissues of the host was observed protruding 
from the parasite’s mouth. 
Frequently the number of young larvae found in a single host much 
exceed the number that ever reach maturity. Single hosts have been 
found to contain 21, 27, 32, 34, and 40 young larvae. In several instances 
1 or 2 of such larvae were found in a stunted condition while the others in 
the same host were normal. In one host a single dead larva was dis¬ 
covered, while the others appeared normal and healthy. In another case 
5 partially developed young larvae (2 of these exceptionally large ones), 
and 4 very small, poorly developed ones were found. At times all the 
young larvae were found dead within the host. Where this occurred the 
cause frequently was found to be hyperparasitism by undetermined 
chalcidoids. 
THE MATURE LARVA 
The mature larva (the lateral aspect of which is shown in PI. 2, C) is 
about 1 millimeter long by 0.5 millimeter thick. It is white, ovoid, bare 
of setae, and with 11 clearly defined body segments. Spiracles are present 
on the second and third thoracic segments and second abdominal segment 
only. In the first abdominal segment, instead of an external spiracle, a 
large discoidal body occurs under the cuticle at the terminus of the 
lateral tracheal branch of this segment. The mouth (PI. 2, D) is a small 
