INSECTA. 



989 



amounts to even fifteen or twenty degrees. In 

 all cases the amount of temperature depends 

 chiefly upon the quantity of respiration. 



We have seen that in a state of repose the 

 acts of respiration become less and less frequent, 

 and that at last they are scarcely perceptible. 

 In like manner the amount of heat generated 

 is proportionate to the diminished number of 

 respirations, and continues to be lessened until 

 the temperature of the insect has very nearly 

 sunk down to a level with that of the atmo- 

 sphere. There are also other circumstances 

 which moderate the production of heat. When 

 the insect is fasting less heat is generated, even 

 during a state of activity, than when the insect 

 is not deprived of its proper quantity of food. 

 The cause of this deficiency seems readily to be 

 accounted for on the consideration that no new 

 material, the product of digestion, is taken into 

 the general circulation, and requires to be assi- 

 milated with the circulatory fluids, and, conse- 

 quently, there is less change in the chemical 

 condition of the fluids at each respiration, than 

 when the animal is taking its full amount of 

 food. Other circumstances also tend to regu- 

 late the amount of heat. When the insect is 

 respiring rapidly, the power and frequency of 

 its circulation are augmented, and not only is 

 there a greater quantity of gaseous expenditure 

 from the respiratory organs, but there is also a 

 greater amount of cutaneous expenditure, which 

 tends to cool down the insect by evaporation 

 from the surface of its body, and diminish the 

 amount of heat developed. This expenditure 

 is so enormous, as we have elsewhere shown,* 

 that it is more than equal to the quantity of 

 solid matter excreted from the alimentary canal 

 in a given time, and, consequently, is a power- 

 ful means of reducing the temperature of the 

 insect. One very marked difference which ex- 

 ists, in respect to the function of respiration and 

 the evolution of heat, between these air-breath- 

 ing invertebrate and the vertebrated animals 

 is, not in their different powers of generating, 

 but of maintaining their temperature. Insects 

 in their low power of maintaining heat closely 

 resemble the true hybernating animals. Any 

 great or sudden change of temperature in the 

 surrounding medium rapidly reduces the tem- 

 perature of the insect. It is thus seen that a 

 reduction of temperature takes place not only 

 at the period of true hybernation, which insects 

 undergo either in their pupa or perfect state, 

 but also during vicissitudes of the season, as 

 well as during natural repose. On the other 

 hand, it is remarkable that the evolution of heat 

 in insects takes place as rapidly as it becomes 

 reduced. Its increase is perceptible by the 

 thermometer within a very few moments after 

 the insect has begun to respire more rapidly. 

 The explanation of this circumstance must be 

 sought for in the peculiar distribution of the 

 respiratory organs, which are extended over the 

 whole body, and aerate the blood in every part 

 of it at the same instant, the result of which is 

 the immediate evolution of a large amount of 

 heat, from the changes that occur in the fluids 



* Phil. Trans. 1837, p. 2. 



in vessels that partake both of the venous 

 and arterial character. Consequently a large 

 amount of heat is liberated instantaneously, 

 whether the oxygen of the atmosphere be ab- 

 sorbed into the circulatory system, or whether 

 the whole of the changes take place, and car- 

 bonic acid be formed in the respiratory struc- 

 tures. The same rapidity with which the heat 

 is evolved from the body accompanies its dimi- 

 nution when the quantity of air inspired is 

 lessened. In confirmation of this view with 

 regard to the production of heat being the result 

 of the chemical changes in the air inspired, 

 there is one remarkable circumstance that can- 

 not be passed over. It is the voluntary power 

 which we have found is possessed by some 

 species of generating heat by means of accelera- 

 ting their respiration. W r e observed this fact in 

 the individuals of a nest of Humble-bees, 'Bom- 

 bus terrestris, which were confined by us for 

 the purpose of watching their habits. Huber 

 formerly noticed that bees are in the habit of 

 incubating on the cells of their pupse, before the 

 perfect insects are developed, and we have had 

 ample opportunities of confirming his observa- 

 tions. It was during the time that we were 

 engaged in watching these habits that we dis- 

 covered that bees possess this voluntary power 

 of increasing their temperature. The manner 

 in which the bee performs her incubatory office 

 is by placing herself upon the cell of a nymph 

 that is soon to be developed, and then begin- 

 ning to respire at first very gradually. In a 

 short time the respirations become more and 

 more frequent, until at length they are increased 

 to one hundred and twenty or one hundred 

 and thirty per minute. The body of the insect 

 soon becomes of a high temperature, and on 

 close inspection is often found to be bathed 

 with perspiration. When this is the case the 

 temperature of the insect soon becomes re- 

 duced, and the insect leaves the cell, and ano- 

 ther bee almost immediately takes her place. 

 When respiration is performed less violently, 

 and consequently less heat is evolved, the same 

 bee will often continue on a cell for many hours 

 in succession. During these observations we 

 have, in some instances, found the temperature 

 of a single bee exceed that of the atmosphere 

 more than twenty degrees. Thus when the 

 temperature of the atmosphere was 73 5 Fahr. 

 that of four female bees, in the act of incuba- 

 tion, was 94 1. On another occasion when 

 the atmosphere was 72 5, a single bee, 

 nursing on a single cell, from which a per- 

 fect insect was developed about eight hours 

 afterwards, afforded a temperature of 92 3, 

 the bulb of the thermometer being placed 

 between the abdomen of the bee and the 

 cell. The insect was then breathing at the 

 rate of one hundred and twenty respirations per 

 minute. In another instance, the temperature 

 of the atmosphere being the same, that of ano- 

 ther bee in the act of nursing was 94 5. 

 This extreme amount of heat was evolved en- 

 tirely by an act of the will in accelerating the 

 respiratory efforts, a strong indication of the re- 

 lation which subsists between the function of 

 respiration and the development of animal 



