OF INSECTS. 
153 
where the esophagus enters the mouth, either in a 
simple opening, in two or three branches, or in a 
numerous series of small ramifications. Its texture is 
more delicate than that of the propulsive portion of 
the organ, and it becomes gradually narrower as it 
approaches the head, (Plate III. fig. 1 ,c,d.) 
The pulsations or alternate contractions and ex- 
pansions of the dorsal vessel, in other words, the 
beating of the heart, vary greatly in number, within 
a given time, in different individuals, and even in the 
same individual, according to its stage of life and the 
temperature. It is most rapid in caterpillars, and 
slowest in the perfect insect. In the former, at an 
ordinary temperature, it has been observed to pulsate 
from thirty to forty-eight times in a minute ; and 
when the heat was increased, the pulsations became 
so rapid and irregular that they could not be counted. 
Much uncertainty still exists with regard to the 
manner in which the blood is conveyed to the diffe- 
rent parts of the body after it is discharged from the 
heart. It is found to pervade the abdominal cavity, 
and to penetrate to all the extremities, without ex- 
cepting the antennse, legs, and wings. Distinct cur- 
rents have been noticed in these members, apparently 
with a well defined course, but no blood-vessels can 
be said to have been any where detected. This 
extra vascular motion of the blood is thought by some 
to be necessary to the adequate performance of the 
corporeal functions in insects, as the tunics of a vessel 
formed round it might interfere with the due deposi- 
tion of oxygen. Nor, as Burmeister states, is the 
