GARDEN PESTS IN NEW ZEALAND 



CHAPTER IV. 



Life Histories of Insects. 



NO doubt owing to the endless assortment of sizes, from mere specks 

 to'giants of a few inches, a widespread idea has arisen, particularly 

 in regard to such insects as have a general resemblance to one another, 

 that the smaller individuals are the younger stages of the larger. 

 Though gradation in size may be a sign of successive ages in certain 

 insects, the presence of functioning wings denotes that growth has 

 ceased; in the case of wingless insects, the characters of maturity may 

 be less conspicuous. Although there may be at times a fairly wide 

 range in size among fully-grown individuals of the onef species, such 

 variation is not due to age, but to certain factors influencing the insect 

 during growth, such as the abundance or scarcity of food supply, and 

 favourable or unfavourable climatic conditions. On the other hand, the 

 sex to which an individual belongs is often responsible for difference in 

 size, males very frequently being smaller than females. Size, therefore, 

 is by no means a sign of age, and the smaller winged insects must not 

 be regarded as the young of the larger ones, no matter how close is the 

 resemblance. 



Insects, with the exception of certain species giving birth to living 

 young, are reproduced from eggs laid by the females; with few excep- 

 tions, the latter take no further interest in, the eggs beyond placing 

 them in surroundings offering the most favourable conditions for their 

 well-being, and a sufficient food supply for the forthcoming young ; each 

 egg is protected by a delicate shell, through which the young insect 

 makes its way on hatching. 



On emerging from the egg, the young insect commences to feed 

 and grow in size, until very soon a stage! is reached when the cuticle 

 or shell becomes too small for the enclosed insect; a fluid then collects 

 between the cuticle and the underlying skin, and a new and more 

 roomv cuticle is secreted by the latter; on this process being completed, 

 the old chitinous covering splits, and the insect withdraws itself. This 

 moulting takes place several times, until the body is fully grown, when 

 the cuticle formed at the last moult is retained by the now adult insect 

 for the rest of its life. 



The different stages through which an insect passes from egg to 

 adult constitute its life history, or life cycle, and the relation of the 

 latter to the seasons, its seasonal history. According to the species, a 

 full twelve months or even more may be necessary for the complete life 

 cycle, or the cycle may be repeated several times within the year; when 

 the cycle occupies twelve months, the insect is single-brooded; but two, 

 three, or four-brooded, etc., when the cycle is repeated two, three, or 



