988 



INSECTA. 



respirations vary, as in Vertebrata, according 

 to the degree of activity and state of excite- 

 ment of the insect. Thus when an insect has 

 been subject to long-continued exertion, the 

 acts of respiration are quick and laborious, as 

 every one must have observed in the larger 

 Bombi, when alighting after a long-continued 

 flight. The contractions and extensions of the 

 abdominal segments are then short and quick, 

 and sometimes so labored that the whole ab- 

 domen is shortened and extended like the 

 flanks and ribs of the race-horse, after a long 

 and severely contested race. The number of 

 respirations, when the insect is in a state of 

 moderate excitement, "varies also in different 

 species as well as in the different states of the 

 same insect, and at different periods. Thus in 

 the, green grasshopper we have noticed from 

 thirty to forty regular contractions in a minute, 

 alternating, at irregular periods, with others 

 more long and deep than the rest. When this 

 insect was much excited the intervals between 

 the long inspirations were longer, and the in- 

 spirations when they occurred were more deep 

 and laborious. When an insect is preparing 

 itself for flight the act of respiration resembles 

 that of birds under similar circumstances. 

 At the moment of elevating its elytra and ex- 

 panding its wings, which are, indeed, acts of 

 respiration, the anterior pairs of spiracles are 

 opened, and the air rushing into them is ex- 

 tended over the whole body, which, by the ex- 

 pansion of the air-bags, is enlarged in bulk, 

 and rendered of less specific gravity, so that 

 when the spiracles are closed at the instant the 

 insect endeavours to make the first stroke with 

 and raise itself upon its wings, it is enabled to 

 rise in the air, and sustain a long and power- 

 ful flight with but little muscular exertion. 

 In the pupa and larva state respiration is per- 

 formed more equally by all the spiracles, and 

 less especially by the thoracic ones. 



The frequency of the acts of respiration 

 seem to bear some relation to the expenditure 

 of muscular energy by the insect in a state of 

 activity. All volant insects respire a greater 

 quantity of air in a given time than terrestrial, 

 and both these in their perfect than in their 

 larva state. Thus in the common hive-bee we 

 have noticed from one hundred and ten to one 

 hundred and sixty contractions of the abdo- 

 minal segments in a minute, while in a less 

 active state, when the insect was entirely un- 

 disturbed, the acts of respiration seldom 

 amounted to one-half that number. In an 

 exceedingly wild and irritable little bee, An- 

 thophora retusa, which dies exhausted from the 

 most violent excitement and exertion, in the 

 course of an hour or two, after being captured 

 and confined during summer, the acts of re- 

 spiration are often performed so rapidly that, on 

 one occasion on which we observed them, they 

 amounted to two hundred and forty in a minute, 

 and of course it was only by the closest atten- 

 tion that their number could be ascertained. 



Next to the pupa state, a state of common 

 repose is that in which insects respire with the 

 least frequency. When a perfect insect or a 



pupa has remained for some time undisturbed, 

 its respiration becomes gradually slower and 

 slower, until at last it is scarcely perceptible. 

 Thus, in the midst of winter, at a temperature 

 of the atmosphere a little below freezing, we 

 were unable to detect more than the very 

 slightest trace of respiration during two or three 

 days, and even at the expiration of fourteen 

 days the quantity of carbonic acid gas formed 

 was very small. But when the insect was re- 

 moved into a much warmer atmosphere it 

 began again to respire more freely, and the 

 quantity of carbonic acid produced in a given 

 time was considerably increased. In a speci- 

 men of Bombus terrestris, which had remained 

 at rest for about half an hour, the respirations 

 had become deep and laboured, and were 

 continued regularly at about fifty-eight per 

 minute. At the expiration of one hundred 

 and forty minutes, during which time the in- 

 sect remained in a state of repose, the respi- 

 rations were only forty-six per minute, and at 

 the expiration of one hundred and eighty mi- 

 nutes they were no longer perceptible. This 

 same insect, when first captured, and in a state 

 of moderate excitement, respired at the rate of 

 one hundred and twenty-five inspirations per mi- 

 nute. We have noticed the like circumstances in 

 a female Sphinx ligustri, which after it had been 

 excited for a short time in flight breathed at the 

 rate of forty-two respirations per minute, while 

 after it had remained at rest about seventy-five 

 minutes respired at the rate of only fifteen per 

 minute. Hence the quantity of air deterio- 

 rated by an insect in a given time depends 

 upon the state of activity and condition of life 

 of the individual. In accordance with the 

 frequency, and consequently the quantity of 

 respiration, such are the results. In accele- 

 rated respiration the circulation of the fluids 

 is increased, and in those conditions, noticed 

 in another part of this paper, in which the cir- 

 culation is accelerated, the acts of respiration 

 are at the same time more frequent. 



The development of heat, which is now found 

 to take place in all insects as in the air-breath- 

 ing vertebrata, depends mainly upon the quan- 

 tity and activity of respiration, and the volume 

 and velocity of the circulation. In Hymenop- 

 tera, in which, as we have seen, the capacity of 

 the respiratory organs is greater than in other 

 insects, and consequently the quantity of air 

 deteriorated in a given time is also greater, the 

 quantity of heat evolved during the process is 

 proportionate to the quantity of respiration. In 

 the larva, in which, in relation to its size, the 

 quantity and energy of respiration are less than 

 in the perfect insect, the quantity of heat deve- 

 loped is also less, so that in the larvae of Hyme- 

 noptera it does not exceed from two to four 

 degrees that of the medium in which the insect 

 is placed ; while in the perfect insect, in a state 

 of little activity, the quantity of heat developed 

 is apparently at its minimum amount at three 

 or four degrees above the temperature of the 

 surrounding medium. But when the same 

 insect is in a state of moderate activity, and 

 consequently is respiring more frequently, it 



