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 expansions 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 different 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 excepting the antennæ, legs, and wings. Distinct currents 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 extravascular 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 deposition of oxygen. Nor, as Burmeister states, is the
