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 antennae, 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 

 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 deposi- 

 tion of oxygen. Nor, as Burmeister states, is the 



