102 



ANALYSIS OF THE ENVIRONMENT 



quantity of such colloids may be a factor in 

 super-cooling. It has been suggested that the 

 water which remains unfrozen at —20° is 

 'bound' to the tissue proteins; but there is at 

 present no way of distinguishing 'bound' water 

 from water which is supercooled from some 

 other cause." 



Not all insects undergo a decrease m 

 percentage of body water in winter. The 

 mound-building ant, Formica exsectoides, of 

 which many individuals retire to points 

 near the water line of the soil before hiber- 

 nating, has practically the same percentage 

 of body water at all times during the year. 

 In fact, hibernation is often associated with 

 a retreat into a more protected situation. 

 The more protected micro-habitats do not 

 reach the freezing points of supercooled 



for long periods are least likely to be im- 

 mobilized by chilling to zero degree Centi- 

 grade. Even acclimatization to higher 

 temperatures of stages in the life history 

 that are normally exposed to low tempera- 

 tures for long periods may not raise the 

 temperature at which chill coma sets in. 

 Later stages of the same insects, stages that 

 ordinarily Hve at higher temperatures, are 

 more easily aflEected by cold and have the 

 temperature at which they pass into coma 

 raised by exposure to warmth (Mellanby, 

 1940). Many poikilothermal animals of the 

 higher latitudes are able to withstand being 

 frozen. Normally, both freezing and thaw- 

 ing take place slowly, and this seems to be 

 important so far as thawing is concerned. 

 There is laboratory evidence that quick 



SPECIES 



Alloeapnia mystic a 

 Allocapnia recta 

 Allocapnio vivipara 

 Taeniopteryx nivalis 

 Allocapnia gronulata 

 Strophopteryx fasciota 

 Perl/ net I a dry mo 

 Isoperia bilineota 

 Neophasganophora capitato 

 A toper la ephyre 

 Perlesta placida 

 Acroneuria arida 

 Acroneuria internata 

 Neoperia clymene 

 Acroneuria abnormis 



NOV 



AUG. 



Fig. 14. Seasonal succession in stone flies of Illinois. The width of the spindle suggests abun- 

 dance. (Redrawn from Frison, ) 



body liquids, and the most protected niches 

 of soil and forest do not normally reach the 

 freezing point of water even in climates 

 such as those of northern Illinois (Bach- 

 metjew, 1907; Payne, 1926; Holmquist, 

 1926, 1931; Dreyer, 1938). 



The development of cold hardiness in in- 

 sects has a species as well as an individual 

 basis. This is illustrated by the seasonal suc- 

 cession of stone flies. About one-third of the 

 species of the order Plecoptera in Illinois 

 emerge as adults, mate, feed, and carry on 

 all essential activities during the coldest 

 months of the year (Frison, 1929, 1935). 

 Certain of these seasonal relations are 

 shown in Figure 14. The racial character 

 of cold hardiness is further illustrated by the 

 fact tliat those stages of arctic insects that 

 are mrmally exposed to low temperatures 



freezing at low temperatures is less harm- 

 ful than is the slower process; but such 

 freezing is presumably rare in nature 

 (Uvarov, 1931; Parker, 1930; Zeuthen, 

 1939). 



Animals frequently develop structures 

 that aid in over\vintering. The gemmules of 

 sponges give an illustration. Fresh- water 

 sponges usually form large numbers of re- 

 sistant gemmules in the autimin and then 

 disintegrate. The gemmules can vdthstand 

 freezing and drying and begin growth 

 anew under favorable conditions. In nature, 

 they normally start to develop in the spring 

 when temperatures rise. Freezing and dry- 

 ing are not always necessary; gemmules of 

 Spongilla have hatched out after two week's 

 exposure to a temperature of 22° C. in the 

 autimm. The accelerating eflPect of exposure 



