100 



ANALYSIS OF THE ENVIRONMENT 



their less conspicuous northward migrations 

 in the spring, are noteworthy, since only in 

 this species are the same individual butter- 

 flies known to make the return journey 

 (Williams, 1930, p. 323; 1938; Beall, 

 1941). Usually, as winter approaches, in- 

 sects of the tree tops or other exposed 

 places migrate, a short distance to less ex- 

 posed niches where they escape full ex- 

 posure to the cold. 



The phenomenon of supercooling appar- 

 ently plays an important role in the cold- 

 hardiness of insects. The body temperature 

 of the adult or juvenile insect, or of eggs, 

 falls, with that of the environment to a criti- 

 cal point at which the temperature re- 

 bounds to a brief equilibrium between the 

 heat of fusion and the radiation of heat into 

 the immediate environment, and remains 

 there until the heat released by freezing 



,B = DEATH 

 Sc^A — Permanent heat stupor 



Temporary heat stupor 

 = Beginning of heat stupor 

 •Supra-optimal zone 

 OPTIMUM 

 ■< Suboptimal zone 



(frozen 

 fluids) 



T2=Criticab state T3 ^^^^^ 

 Fig. 13. Temperature relations of an insect. (Redrawn from Uvarov, after Bachmetjew.) 



The overwintering of insects has been 

 much studied both by ecologists and by 

 physiologists. Here, as elsewhere, it is hard 

 to separate the work and interests of these 

 two groups. Usually, insects from exposed 

 positions, such as those of tree tops, migrate 

 a short distance to protected micro-habitats 

 and so escape from the full impact of win- 

 ter conditions. A number of adaptive 

 processes take place as temperature falls: 



(1) The activity of the insects decreases; 



(2) production of metabolic water lessens; 



(3) the percentage of salts and colloids in 

 body liquids increases; (4) other colloidal 

 relations with water may change. From the 

 integration of insect behavior with biophysi- 

 cal and biochemical processes, most insects 

 pass the winter without being frozen, even 

 though they are living in a continental cli- 

 mate in the middle latitudes. 



The temperature relations of insects in 

 such climates are summarized in Figure 13. 



becomes dissipated; then the temperature 

 drifts downward again to stable equilibrium 

 with the environment. The freezing tem- 

 perature in insect bodies is not closely 

 related either to the limit of liquid under- 

 cooUng or to what is sometimes called the 

 rebound temperature. In insects, supercool- 

 ing before ice formation starts within the 

 body and carries down from a few degrees 

 below zero to —40° or —50° C. (Salt and 

 Mail, 1943). 



It is difficult even to approximate the 

 true freezing temperature of an insect. The 

 rebound point is often taken as the freez- 

 ing point, a concept now known to be 

 erroneous, since the two may differ by as 

 much as 25 degrees. Water in bulk under- 

 cools several degrees; it undercools still 

 more in an emulsion, and when subdi- 

 vided, as a fog, it has been undercooled to 

 — 155° C, in the laboratory. Natural fogs 

 occur with temperatures of —25° C. to 



