INTRODUCTION ! PART TWO 57 



adaptation. The males have antennae especially modified for grasping 

 the female during copulation. 



Despite these features which they have in common, the two orders 

 present a profound contrast. 



First of all the feather louse undergoes no metamorphosis. When it 

 hatches from the egg, a feat it accomplishes by pushing up the cap which 

 opens like a lid, the nymph or young louse which emerges is more or 

 less a miniature edition of the adult — minus the sexual organs. 



It reaches maturity by a series of three moults, that is to say it 

 periodically casts off its integument for which it has grown too big. 

 Each time it changes its skin in this manner it becomes a httle more like 

 the perfect insect. Its whole life-history from egg onwards is passed 

 upon the host, and from the day it hatches it can chew feathers. The 

 feather louse's world is the hot, fidgeting body of the bird, with which 

 its fate is indissolubly linked. The reproduction of the host means a 

 future for the louse and the death of the host spells its inevitable doom. 



Fleas on the otherhand, have a complete metamorphosis (Plate 

 XVIII). The larva, which develops inside the egg, hatches by ripping up 

 the egg-shell with a special spine situated on the front of the head. The 

 sort of legless caterpillar which emerges does not remotely resemble a 

 flea, and at this stage chews its food rather Hke a feather louse. After a 

 series of moults it changes into a pupa, or resting stage, from which, in 

 due course, the perfect insect emerges. Unlike the larval stages of the 

 feather louse, those of the flea are free and are generally passed in the 

 host's nest. 



In many cases fleas themselves only spend a limited amount of 

 time on the body of the host, and should the bird die they can survive 

 for days, weeks, or even months, hiding in the nest or some appropriate 

 crack or crevice. They can also live for a long period after emerging 

 without food, but ultimately they are compelled to partake of a blood 

 meal or perish without reproducing themselves. 



Structurally the adult insect also presents a considerable contrast. 

 Fleas are flattened from side to side, essentiafly an adaptation to 

 life in fur. Only a very fat man who has once been thin can apprec- 

 iate the advantage of not having to turn sideways to get through a 

 gap, especiaUy if he happens to be in a hurry. On the host, fleas are 

 nearly always in a hurry. 



Feather lice on the other hand are flattened from above downwards. 

 Their life depends on being able to cling closely to the feathers, or, 



