Fig. 383.—Parts of a vein of the cockroach, showing the nerve (n) by the side of the trachea (tr); c, blood corpuscles.—After Moseley.
Moseley found, owing to the large size and number of the corpuscles, that the circulation of the blood in the wings of insects is most easily observed in the cockroach, especially the hind wings. As seen in Moseley’s figure, the blood flows outward from the body through the larger veins (I and II) of the front edge of the wings, which he calls the main arteries of the wings, and more generally returns to the body through the veins in the middle of the wing; the blood also flows out from the body through the inner longitudinal veins (those behind vein IV), and the blood is also seen to flow through some of the small cross-veins. Fig. 383 shows one of the main trunks during active circulation. The corpuscles change their form readily, “the spindle-shaped ones doubling up in order to pass crossways through a narrow aperture.... In the irregularly formed corpuscles, which seem to represent leucocytes amœboid movements were observed.... Corpuscles pass freely above and under the tracheæ, showing that these latter lie free in the vessels.” The hypodermis lining the vessels is best seen in the small transverse veins.
The pulse or heart-beat of insects varies in rapidity in different insects, rising at times of excitement, as Newport noticed in Anthophora retusa, to 142 beats in a minute.
When an insect, as, for example, a tineid caterpillar, has been enclosed in a tight box for a day or more, the pulsations of the heart are very languid and slow, but soon, on giving it air, the pulsations will, as we have observed, rise in frequency to about 60 a minute, Herold observed 30 to 40 in a minute in a fully-grown silkworm, and from 46 to 48 in a much younger one. Suckow observed but 30 a minute in a full-grown caterpillar of Gastropacha pini, and 18 only in its pupa.
In a series of observations made by Newport on Sphinx ligustri from the fourth day after hatching from the egg until the perfect insect was developed, he found that before the larva cast its first skin the mean number of pulsations, in a state of moderate activity and quietude, was about 82 or 83 a minute; before the second moult 89, while before the third casting it had sunk down to 63; and before its fourth to 45, while, before leaving its fourth stage, and before it had ceased to feed, preparatory to pupating, the pulse was not more than 39. “Thus the number gradually decreases during the growing larva state, but the force of the circulation is very much augmented. Now when the insect is in a state of perfect rest, previously to changing its skin, the number is pretty nearly equal at each period, being about 30. When the insect has passed into the pupa state it sinks down to 22, and subsequently to 10 or 12, and after that, during the period of hibernation, it almost entirely ceases. But when the same insect which we had watched from its earliest condition was developed into the perfect state in May of the following spring, the number of pulsations, after the insect had been for some time excited in flight around the room, amounted to from 110 to 139; and when the same insect was in a state of repose, to from 41 to 50. When, however, the great business of life, the continuation of the species, has been accomplished, or when the insect is exhausted, and perishing through want of food or other causes, the number of pulsations gradually diminishes, until the motions of the heart are almost imperceptible.” Insects, then, he remarks, do not deviate from other animals in regard to their vital phenomena, though it has been wrongly imagined that the nutrient and circulatory functions are less active in the perfect than in the larval condition.
The heart of a larval Gastrus equi taken the day previous from a horse’s stomach beat from 40 to 44 times a minute (Scheiber); while Schröder van der Kolk observed only 30 beats in the same kind of maggot.
In the larva of Corethra, while at rest, the heart contracts from 12 to 16 or 18 times a minute, but when active the number rises to 22. The systole and diastole last from 5 to 6 minutes. (Dogiel.)
Temperature also affects the pulsations, as they increase in frequency with a rise and decrease with a fall in temperature.
Influence of electricity.—The influence of electricity on the action of the insect’s heart, from Dogiel’s experiments, is such as to cause an acceleration in the frequency of the beats, while an increase in the strength of the electric currents either diminishes the frequency of the beats or entirely stops the heart’s action. A violent excitation with the induction current causes a systole when the heart’s action has stopped for a long time; and if the excitation lasts uninterruptedly, then the contractions after a while become noticeable, according to the strength of the current. In such a case there are, however, interruptions in the regularity, strength, and order of the contractions. (Kolbe.)