ECOLOGY OF INSECTS 403 



many cases death follows as the rapid nemesis of repro- 

 duction. But though the adult life is often very short, 

 the total life may be of considerable length, as in some 

 Ephemerids, which in their adult life of winged love- 

 making may be literally the flies of a day, while their 

 aquatic larval stages may have lived for two years or more. 



The relation between the annual appearance of certain 

 insects and that of the plants which they visit, the habits 

 of hibernation in the adult or larval state, the occasional 

 " dimorphism " between winter and summer broods of 

 butterflies, should be noticed. 



The prolific multiplication of many insects may lead to 

 local and periodic increase in their numbers, but great 

 increase is limited by the food-supply and the weather, 

 by the warfare between insects of different kinds, by the 

 numerous insects parasitic on others, by the appetite of 

 higher animals — fishes, frogs, ant-eaters, insectivores, and, 

 above all, birds. 



There is a great variety of protective adaptation. The 

 young of caddis-flies are partially masked by their external 

 cases of pebbles and fragments of stem ; many cater- 

 pillars and adult insects harmonise with the colour of their 

 environment ; leaf-insects, " walking sticks," moss-in- 

 sects, scale-insects, have a precise resemblance to external 

 objects which must often save them ; a humming-bird 

 moth may resemble a humming-bird ; many palatable 

 insects and larvae have a mimetic resemblance to others 

 which are nauseous or otherwise little likely to be meddled 

 with. Many insects may be saved by their hard chitinous 

 armour, by their disgusting odour or taste, by their de- 

 terrent discharges of repulsive formic acid, etc., by simula- 

 tion of death, by active resistance with effective weapons. 



Many flowers depend for cross-fertilisation upon in- 

 sects, which carry the pollen from one to another. Many 

 insects depend for food on the nectar and pollen of flowers. 

 Thus many flowers and insects are mutually dependent. 

 But many insects injure plants, and many plants exhibit 

 structures which tend to save them from attack. On the 

 other hand, there may be " partnerships " between insects 

 and plants — as in the " myrmecophilous " (ant-loving) 

 plants, which shelter a bodyguard of ants, by whom they 



