^58 Comparative Animal Physiology 



The mechanisms of perception of thermal gradients by insects are not well 

 known, and some temperature selection may well depend on the absolute 

 temperature and hence on the activity of the nervous system. There are, 

 however, peripheral thermal receptors in some insects. In crickets the antennae 

 perceive air temperature and the tarsi perceive ground temperature, while 

 mouthparts and cerci also are temperature sensitivc^*^ In grasshoppers which 

 orient in the sun, receptors appear to be scattered over the whole body.^° The 

 antennae are more sensitive to thermal stimuli than other regions in most 

 insects. The blood-sucking bug (Rhodnitis) is attracted to a mammal partly 

 by smell, but from short range it is attracted by thermal gradient in the air."^' 

 The eyes are not used, but the bug waves its antennae about as if comparing 

 the temperature gradient, and, if the antennae are removed, the insects become 

 torpid and unresponsive to warm objects. Bedbugs and phasmids also have 

 warmth receptors on the antennae. A positive reaction to warmth is important 

 also in enabling a mosquito to locate its prey. The physiology of thermo- 

 reception in insects has not been studied, and there seems to be no evidence 

 for cold receptors. 



The manner of orientation and aggregation in thermal gradients has been 

 discussed.**^ In no insect is orientation direct, as it may be to light, but rather 

 it is achieved by trial and error. There are random movements, particularly 

 of antennae, so that the response appears to be a "reflex pursuit of the antennae 

 into a zone of optimal stimulation. "^'^•^ Stimulation is by heat gradient, not by 

 radiant heat, and only in the parasites of warm-blooded animals can heat act 

 as a token stimulus. Selection then depends more on activity than on direction 

 of movement. The activity of blowflies kept 12 hours at different temperatures 

 was maximal at 30°, which was in the middle of the selected range, whereas 

 if the temperature was raised from 1° to 45° during 6 hours, peaks of activity 

 were noted at 20° and at 42°. Thus, although there might be changes in 

 activity associated with temperature change, the equilibrium activity seemed 

 correlated with temperature selection. ^-'•' When presented with two sources 

 of heat stimulation, insects such as lice circle between the two sources and then 

 go toward one of them.'^'' 



In insects in general, body temperature is normally that of the environment, 

 when allowance is made for evaporation, but slight regulation against tempera- 

 ture stress can be brought about by (1) altering water loss or retention; (2) 

 using metabolic heat as in exercise; (3) clustering, fanning, and transporting 

 water, as by colonial species; (4) orientation of position and absorption of 

 radiant heat; and (5) aggregation in regions of selected temperature. Loco- 

 motion toward a heat source, best seen in parasites of homoiotherms, is not 

 direct and appears to depend on comparison of thermal gradients by receptors, 

 particularly on the antennae. 



Body Temperature of Reptiles. Reptiles are the largest poikilotherms, and 

 from them the homoiothermic animals arose. The change in body temperature 

 of reptiles with change in environmental temperature is slow because of their 

 high specitic heat (Fig. 92), and numerous adaptations anticipate true tempera- 

 ture regulation. The rectal temperature in large pythons increases with 

 activity, and the body temperature of a female python incubating eggs was 

 33° in air of 31°, whereas between the coils the temperature was higher 

 (33.5°).'" Reptiles are much less dependent on humidity than are amphibia. 



