114 MAUD D. HAVILAND 
longitudinal, and lateral muscles of the posterior segments 
(fig. 9). 
The circulatory system calls for no particular comment. 
In the tracheal system of the fourth instar larva there are still 
seven pairs of open spiracles, for the eighth (mesothoracic) does 
not become functional until metamorphosis. The first spiracle 
is situated between the first and second segments, and the 
second on the posterior side of the third segment, while the 
remainder are on the five following segments. In addition, 
two rudimentary stigmatic trunks can sometimes be seen on 
the ninth and tenth segments, and the anterior one is occasion- 
ally visible during the third instar. It appears that these 
trunks are never functional, and they were not always apparent 
in the larvae examined. [mms (11) has described vestigial 
stigmatic trunks on the eleventh segment of the full-grown 
larva of Aphycus melanostomatus, which has nine 
pairs of functional spiracles. These do not appear in the 
Lygocerus larva, in which the spiracles have evidently been 
reduced in number from behind forwards. The aborted 
trunks of segments nine and ten are probably vestiges inherited 
from an ancestral form with ten open spiracles. The rest of the 
tracheal system differs from that of the preceding stage only 
in the greater calibre and more elaborate ramifications of the 
tubes. It should, however, be remarked that there is no ana- 
stomosis of the tracheal branches of the two sides of the body, 
such as Seurat (26) describes in certam Ichneumonidae and 
Braconidae (fig. 7). 
The nervous system consists of two supra-ocsophageal 
ganglia, united by a broad commissure, and connected with the 
sub-oesophageal ganglion by two short, thick cireum-oesophageal 
commissures. The ventral nerve cord contains eleven ganglia. 
The four anterior are well marked ; the five following are less 
distinct, and appear as a wide, slightly-segmented band. The 
cord terminates in a bulbous swelling, composed of two 
ganglia, that of the eleventh segment being fused with that of 
the tenth (fig. 12). 
The genital organs lie above the mid-gut on either side as 
