48 
Psyche 
[June 
but with the humeri of the thorax as well. The head of 
the major of texanus can be lowered until its long axis is 
at right angles to the long axis of the rest of the body. Al- 
though many majors die in this position it is rarely as- 
sumed by the living insect, presumably because in such 
a posture the anterior edge of the head is so far below the 
level of the coxae that the insect would have to stand on 
tiptoe to keep the mandibles clear of the substrate. Con- 
versely, the head of the major can be raised until its long 
axis is parallel to that of the rest of the body. Under such 
circumstances the rim of the cephalic disc lies at the same 
level as the dorsum of the thorax. But in this position the 
head of the major does not occlude the passage for there 
is then a considerable space below the ventral surface of 
the head. To occlude the passage the major must lower 
the head until its anterior edge is on or near the floor of 
the passage. But when the head of the major is in this 
position, with its long axis forming an angle of approxi- 
mately 120° with the long axis of the rest of the body, 
the posterior rim of the cephalic disc is well below the 
level of the humeral angles. Thus if a major of texanus is 
to occlude a passage, this passage must be large enough to 
accommodate not only the cephalic disc but also the humeral 
angles above and behind it. For if the head is lowered 
enough to form an effective barrier there is no way in which 
the humeral angles can be brought down to the level of 
the posterior rim of the cephalic disc. It would appear, 
therefore, that it is physically impossible for the major 
of texanus to occlude a passage whose size closely approxi- 
mates that of the cephalic disc. 
It may be readily appreciated that the situation just de- 
scribed is wholly different from that which marks the 
major of Colobopsis. In the cork-shaped head of the major 
of Colobopsis maximum occlusion occurs when the long 
axis of the head is parallel to that of the rest of the body. 
In this position the truncated anterior face of the head 
is presented to the exterior. Since the diameter of this 
truncated portion exceeds that of any other part of the 
Explanation of Plate 3 
Cryptocerus texanus, dorsal view of the major. 
