16 THE HESSIAN FLY IN THE UNITED STATES. 
of hatching to the location under the sheath. Its size is slightly larger 
than the egg from which it hatches, and it presents thirteen segments, 
of which the first constitutes the head. It is distinguished from the 
second form principally by the presence of two triangular, fleshy, ear- 
like appendages, lightly incurved below on each side, and comparable 
to tentacles; the anterior buccal border is trilobed, and beneath is the 
mouth in the form of a small triangular opening. The last segment is 
lightly emarginate posteriorly, and each of the two lobes thus formed 
carries four setiform papillae. 
The second larval form, which is fixed to the stalk and is the one 
usually described by writers, is, when grown, 3 millimeters long by 
a little less than 1 millimeter wide, of a flattened, cylindrical form, and 
so transparent that the internal organs are easily seen. There are 
twelve segments beside the head, which is small and more or less 
retracted. The anterior end is narrowed and usually somewhat bent. 
The posterior end tapers, is bluntly rounded, and with two lobes on 
the posterior segment. The segments are but slightly indicated exter- 
nally, but are plainly marked by the internal masses of adipose tissue 
arranged in series along each side, as well as the respiratory organs, 
the spiracles being plainly visible under the microscope as minute 
openings in rounded yellowish tubercles (Plate II Be). -The mouth- 
parts are indistinct (Plate II B&), and the sternal spatule is, until the 
larva is ready to pass to the next form, either entirely wanting or 
inconspicuous. The digestive, nervous, tracheary, and adipose systems, 
which are very plainly visible in the larvae, have been described in 
particular by Marchal, whose paper should be studied for details in 
this regard. 
The third larval form — that inclosed within the puparium — is dis- 
tinguished especially by the development of the sternal spatule or 
" breastbone." In other respects there is little difference anatomically 
from the preceding, but it is a quiescent not a feeding stage. The 
sternal spatule, which becomes conspicuous at this stage, is a horny 
structure projecting from between the first and second body segments, 
and is provided at its anterior extremity with two pointed spurs. The 
bifurcate form of this spatule is used by Marchal (71) to separate this 
species from avence, in which there is but one point. The use of this 
organ, which projects forward under the first segment and rests against 
its ventral surface, has been a subject of considerable discussion. The 
explanation which is best supported was proposed by Enock and is 
sustained by Marchal (71). This is that the spatule is used by the 
larva to reverse its position in the puparium so that, whereas the larva 
rests at first with its head downward and toward the roots of the 
plant, it rests, after turning, with its head upward and toward the 
upper part of the plant, a position which has obvious advantages when 
Ave consider the direction in which the pupa and imago must escape. 
The pupa when first issued from the puparium is white, but acquires 
