AN ECONOMIC ENTOMOLOGY. 



before it is ready to change to a pupa. I have confined several 

 larvae in vials, closed them tightly with rubber stoppers, and sup- 

 plied a small amount of food. In these dry vials larv^ae have 

 lived for months, in one case more than a year, and apparently 

 as a mere pastime they moulted at intervals, so that in one vial 

 there were eight or ten cast skins, more than doubHng the ordi- 

 nary number of changes. Withal the insects did not increase in 

 size, simply because no more food was supplied. 



Where the metamorphosis is complete, there hatches from 

 the egg a grub, caterpillar, slug, maggot, or other crawling or 

 wormlike creature, quite unlike the parent. Growth continues, 

 as before, by moulting, until the full size has been attained, and 

 then feeding ceases. The larva now seeks some retired spot, 

 sometimes spinning a silken covering termed a cocoon^ and then 

 changes to a chrysalis or pupa. The term chrysalis is used in 

 the butterflies, where it is free and often prettily ornamented, and 

 the term pupa in most other cases. In this stage it is inactive 

 and rests quietly for some time. The character of the future in- 

 sect is usually more or less clearly defined, but occasionally it 

 resembles neither the larva that formed it nor the adult insect 

 which is to come from it. In due time the adult emerges as a 

 butterfly, moth, beetle, fly, or bee, and no trace remains of the 

 previous larval condition. Insects never grow after they reach 

 the adult stage, but remain of exactly the same size as when 

 first they emerged from the pupa. Thus, as has been indicated 

 in the previous chapter, large flies are never the parents of small 

 flies, nor do small flies ever grow to become larger flies. 



This cycle of change in an insect is termed its life history, which 

 it is sometimes very difficult to work out. Scarcely two species 

 are alike in habit during all their lives, and a complete knowledge 

 of the full life cycle and the habits in all stages is essential to an 

 intelligent appHcation of remedial or preventive measures. 



It is often possible to reach an insect easily in one stage, or at 

 one period, where it is naturally protected during all others. So 

 we may, sometimes, by a little change in farm practice, or by 

 taking advantage of some preference on the part of the insect, 

 avoid injury. To ascertain all these facts and to suggest the 

 treatment indicated, is the work of the economic entomologist. 

 Watching the transformations of an insect is a delightful task, 



