274 ARTHROPODS. 



may be of considerable length, witness 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 caterpillars 

 and adult insects harmonise with the colour of their environ- 

 ment ; leaf-insects, " walking sticks," moss-insects, scale- 

 insects, have a precise resemblance to external objects which 

 must often save them ; a humming-bird moth closely resem- 

 bles a humming-bird, many palatable insects and larvae have 

 a mimetic resemblance to others which are nauseous or other- 

 wise little likely to be meddled with. Many insects may be 

 saved by their hard chitinous armour, by their disgusting 

 odour or taste, by their deterrent discharges of repulsive 

 formic acid, etc., by simulation of death, by active resistance 

 with effective weapons. 



Many flowers depend for cross-fertilisation upon insects 

 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— witness the 

 " myrmecophilous " (ant-loving) plants which shelter a 

 bodyguard of ants, by whom they are saved from un- 

 welcome visitors. And again, the formation of galls by 

 some insects which lay their eggs in plants, and the insect- 



