CERTAIN PARASITIC INSECTS. 89 
endeavoring to ascertain the limits of natural groups, all the 
organs collectively, as well as the internal anatomy and the 
embryology and metamorphosis of insects, before we can 
hope to obtain a natural classification. 
The family of bird lice isa very extensive one, embracing 
many genera, and several hundred species. One or more 
species infest the skin of all our domestic and wild mammals 
and birds, some birds sheltering be- m ‘Fig. 17. 
neath their feathers four or five spe- 
cies of lice. Before giving a hasty 
account of some of our more com- 
mon species, we will give a sketch 
of the embryological history of the 
lice,* with especial reference to the 
structure of the mouth parts. 
The eggs (Fig. 17, egg of Pedicu- 
lus capitis) are long, oval, somewhat 
pear-sh:ped, with the hinder end 
somewhat pointed, while the ante- 
rior end is flattened, and bears little 
conical micropyles (m, minute ori- 
fices for the passage of the sperma- 
tozoa into the egg), which vary in 
form in the different species and Embryo of the Louse. 
genera; the opposite end of the egg is provided with a few 
bristles. The female attaches her eggs to the hairs or feath- 
ers of her host. 
After the egg has been fertilized by the male, the blasto- 
derm, or primitive skin, forms, and subsequently two layers, 
or embryonal membranes, appear; the outer is called the 
amnion (Fig. 17, am) (though as Melnikow states, it is not 
homologous with the amnion of vertebrates), while the inner 
* Fo rm 4z +h y EAE TE nt y foa Ps ee, B Professor Nico- 
laus Metnow's ^ th Embryonal Develop tof Insects " in Wiegmann's 
Naturgeschichte, 1859, p. 136. 
AMER. NATURALIST, VOL. IV. 12 
