124 FLEAS, FLUKES AND CUCKOOS 



kept at 33°G. only live a few days. Temperature of the host, therefore 

 affects not only the length of the life-cycle, but also the survival of the 

 young, and may be one of the limiting factors in the establishment of a 

 louse on a new host (see discussion under host specificity, p. 137). 



When the nymph, within the egg^ is ready to hatch it begins to suck 

 in air through its mouth; this air passes through the alimentary canal 

 and accumulates in the egg-shell behind the nymph. After about five 

 minutes of this sucking action the pressure of the air behind the nymph 

 becomes so intense that the cap or operculum of the egg is forced open. 

 During the next twenty minutes the nymph frees itself from the egg 

 shell by muscular contractions, expansion of the abdomen and further 

 pumping of air. It can at once move freely about on the feathers, but 

 food (which is the same as that of the adult) is not taken until several 

 hours after hatching. 



The feather lice, unlike the fleas, have no metamorphosis. The 

 nymph which emerges from the egg resembles the adult in habits and 

 general body form, differing in its smaller size, absence of colour, un- 

 developed sex organs, and certain other morphological details. The 

 nymph sheds its entire skin three times before reaching the adult state. 

 In Columbicola each stage lasts from six to seven days, and after each 

 moult the nymph becomes successively larger, darker and more like 

 the adult. Nothing is known about the length of life of the female or the 

 total number of eggs she lays. But far fewer eggs are required by ecto- 

 parasites which spend generation after generation on one individual 

 bird, than by those in which the young are faced with the risky business 

 of finding a new host after hatching. 



Host and Parasite 



Population Size of Parasite and Effect on Host. The number of lice (or 

 population size) found on any one bird varies considerably from indi- 

 vidual to individual, even in the same species. This variation does not 

 seem to be entirely seasonal; a number of birds of the same species can 

 be examined at the same time of year, and some may be lightly infested, 

 some heavily infested and some altogether louseless. Certain species, for 

 example rooks and crows and some of the waders, have a higher propor- 

 tion of both infested individuals and individual lice, but even amongst 

 these, lice-free birds may be found. A curlew from Ireland, in excellent 



