606 CAROLINE B. THOMPSON AND THOMAS E. SNYDER 
The early development of the third-form nymphs (under 4 
mm.) has not been traced. Their resemblance to the nymphs 
of workers is so close that it would be very difficult to recognize 
one from the other in early stages in which the adult characters 
necessary to distinguish them have not yet appeared. To illus- 
trate, three important adult characters which separate the third 
form from the worker are: the number of antennal segments, 
the thickness of the head chitin, and the fusion or non-fusion 
of the three embryonic fundaments of the female reproductive 
system. It is obvious that these cannot be used in studying 
young nymphs in all of which the antennal segments are incom- 
plete, the chitin thin, and the parts of the reproductive system 
separate. Further detailed study, however, may develop means 
for distinguishing the very youngest nymphs of these two castes. 
The nymph of the third form. 'The nearly mature nymph of 
the third form, 5 to 6 mm. long (fig. 6), has a pure white head and 
body, a thorax devoid of either wings or wing pads, seventeen 
or eighteen antennal segments, and compound eyes that are so 
small and contain so little pigment that they are practically 
invisible in the living or unstained specimen, and are seen only 
with the microscope after staining. Genital appendices are 
present on the ninth abdominal segment in both sexes. These 
characters will serve to distinguish the third-form nymph from 
those of the first and second form. 
The third-form nymph of 5 to 6 mm. may be distinguished by 
the following characters from the adult worker, 5.5 mm. (fig. 7), 
which, it will be recalled, is also wingless, using for this comparison 
unstained specimens and a low-power lens: 1) the color of the 
head—third-form nymph: white, worker: yellow, due to the 
thicker chitin; 2) the color and general appearance of the abdo- 
men—third-form nymph: opaque or dense creamy white, firm 
texture, long, tapering, intestinal contents not woody and not 
visible, worker: transparent or glistening white, loose texture, 
shorter, blunt, intestinal contents woody, and visible as a dark 
mass; 3) the number of antennal segments—third-form nymph: 
seventeen to eighteen, worker: sixteen. See table 1. 
In all three reproductive forms the dense creamy white or 
opaque abdomen is in marked contrast to the partly transparent 
