OVERNINTERING INSECTS: 
     
     ADAPTION FOR SURVIVAL 
     
    
   
   
    
     Insects inhabit almost every part 
     
     of the earth, experiencing widely 
     
     varying conditions of light, tem- 
     
     perature, precipitation and of kinds 
     
     and amounts of vegetation.‘ Over most 
     
     of the continental land masses seasonal 
     
     changes such as winter or long droughts 
     
     limit insect activity to certain 
     
     favourable times of the year. Insects 
     
     must, therefore, be able to endure 
     
     somehow the unfavourable periods. In 
     
     our northern temperature regions the 
     
     unfavourable period is winter; how do 
     
     insects survive the low temperatures 
     
     and the deep snows of Canadian winters? 
     
    

    
     A great many insects can stop their 
     
     growth and activity at some stage in 
     
     their life cycle by entering a kind of 
     
     dormancy which entomologists call 
     
     diapause. Insects in diapause have 
     
     an altered body chemistry which permits 
     
     them to survive food scarcity and fat 
     
     depletion. Any stage of the life cycle 
     
     may be used for diapause in different 
     
     species, but any one species will 
     
     always enter diapause in the same stage 
     
     of development each year. For many 
     
    

    
     aphids, that stage is the egg; for 
     
    

    
     ' some moths, like the European cornborer 
     
     it is the larva; for other moths, such 
     
     as the cecropia, it is the pupa; for 
     
     still other insects the adult stage is 
     
     utilized. . _. 
     
    

    
     While the diapause itself is a 
     
     complex physiological event as yet 
     
     not well understood, its onset is 
     
     preceded or accompanied by specific 
     
     behavioral patterns. Eggs are 
     
     deposited in favourable locations; 
     
     cocoons are soun; larvae, pupae and 
     
     adults enter the soil or find shelter 
     
     in appropriate places, as though 
     
     knowing of hard times ahead, and 
     
     preparing for them. However, it is 
     
     quite clear that insects do not 
     
     anticipate the conditions (cold 
     
     temperatures; lack of food) which 
     
     diapause enables them to survive; 
     
    

    
     ‘ rather, the insects respond to a 
     
     biological "clockwork" mechanism 
     
     which matches the insects' 
     
     behaviour to the-seasonal cycle. In 
     
     latitudes such as ours, the "clock" 
     
     is run by the photoperiod, or 
     
     changing day length; as days shorten 
     
     in autumn, the insects' behaviour 
     
     and body chemistry change towards 
     
     that of the dormant state. These 
     
     changes are brought about in many 
     
    
   
   
    
     insects by the secretion of a 
     
     dispausing hormone from nerve cells 
     
     in the insect's "throat". 
     
    

    
     While the cocoon may protect 
     
     against drying out, and while 
     
     shelters in soil, under stones, 
     
     in rotten logs 0r amongst plant 
     
     debris may help the diapausing 
     
     insect to avoid extremes of cold, 
     
     few of our local outdoor insects 
     
     can evade temperatures that drop 
     
     below 0°C. The basis of insect 
     
     body fluids is water; water 
     
     freezes at 0°C and ice formation 
     
     in living cells is normally fatal. 
     
     How, then, can insects whose body 
     
     temperatures follow that of the 
     
     environment, survive long periods 
     
     of subzero temperatures? Studies 
     
     related to this question of cold 
     
     hardiness have shown two 
     
     interesting phenomena. Firstly, 
     
     it is clear that the body fluids 
     
     of many insects can be supercooled; 
     
     that is, they can remain liquid at 
     
     temperatures well below the freezing 
     
     point in the absence of suitable 
     
     "nucleating centres” at which 
     
     crystallization into ice is initiated. 
     
     Secondly, and perhaps not unrelated 
     
     to the first observation, hibernating 
     
     insects have glycerol in their body 
     
     fluids. We have learned to add 
     
     glycerol to human blood to preserve 
     
     it frozen in hospitals. Insects add 
     
     it to their blood by manufacturing 
     
     it from their stored food, in time 
     
     for winter. In effect, they provide 
     
     their own antifreeze. Not to be 
     
     wasteful, they convert it back to 
     
     food again when post-diapause 
     
     activity is resumed. 
     
    
   
   
    
     Insect physiologists ask, "Is 
     
     there a causal relationship between 
     
     diapause and cold hardiness?" In 
     
     general, they tend to answer 
     
     themselves in the negative. Insofar 
     
     as the phenomena are now understood 
     
     it is believed that diapause has the 
     
     attributes (inactivity, cessation of 
     
     feeding) for favor Survival, whereas 
     
     the factors that confer cold hardiness 
     
     are the Supercooling potential and the 
     
     presence of glycerol. The concurrence 
     
     in timing of these phenomena allow 
     
     insects to pass the long northern 
     
     winters unharmed and to greet the 
     
     succeeding spring in the fall vigour 
     
    
   
   
    
     f their 5 ecies. 
     
     0 p Lawson Drake 
     
    
   
   
    
     Biology Dept. 
     
     U.P.E.I.