28 ENTOMOLOGY FOR MEDICAL OFFICERS 



the female is furnished with an ovipositor, and the male with 

 paired hooks or claspers (Fig. 23). 



The egg as a rule is large, and contains much food-yolk. 

 In a few insects the egg is hatched within the mother, and 

 the young is born as a larva ; but as a rule the eggs are laid 

 as such — singly, or in masses, or in capsules ; sometimes 

 haphazard, but often in an instinctively chosen place on or 

 near food suitable for the larva. There are many insects, again, 

 whose instincts lead them to make for their larvae housing 

 and nursing arrangements that look like the outcome of 

 protracted foresight. 



It is quite exceptional for the new-hatched young to be 

 like the parent : such is the case only with some wingless 

 insects. The young is usually a larva (often with no 

 resemblance to the adult) which passes through a prolonged 

 term of post-embryonic development, or metamorphosis, 

 before it becomes adult. 



Metamorphosis. — The post-embryonic changes, or meta- 

 morphoses, may be gradual or abrupt. Gradual meta- 

 morphosis is known as Hemimetabolous or "Incomplete" 

 Metamorphosis. Here the larva is recognisably like the 

 adult, and differs from it, to external view, chiefly in having 

 no wings, and perhaps a larger head and shorter antennae. 

 In the course of growth the young insect periodically moults, 

 and at each moult it appears a little more like the adult, 

 until at last the wings are fully formed and sexual maturity 

 is attained. The wings in this case first appear as external 

 buds, and when these rudiments are fairly advanced the 

 larva is sometimes spoken of as a nymph. 



Abrupt metamorphosis is known as Holometabolous or 

 " Complete " Metamorphosis. Here the larva (Lat. larva = mask) 

 is often utterly unlike its parents, differing from them not 

 only in the proportions of the body and the absence of 

 wings, but also perhaps in the whole manner of life ; the 

 food may be of a different kind, and consequently the 

 mouth-parts may be of an entirely different type. The 

 larva eats voraciously, puts on fat, grows and moults, until 

 it has reached its full size (or exhausted its food-supplies), 

 when it ceases to feed and becomes more or less quiescent. 

 The dormant insect is now a pupa (Lat. pupa = z. puppet or 



