I I 2 
Book-lice and Bark-lice; Biting Bird-lice 
there is a peculiar little chitinized structure, which may be called the 
oesophageal sclerite (Fig. 145). This structure is situated in the floor of 
the pharynx (forward end of the oesophagus), and has some special function 
in connection with the peculiar food-habits. It was first described by Bur¬ 
gess, and was for a long time supposed to be peculiar 
to the book-lice alone. But, in a study of the mouth 
structure of the biting bird-lice (Mallophaga), I found 
an almost identical oesophageal sclerite in thirteen out 
of the twenty-two genera of the Mallophaga. On 
the basis of this common possession of a curious 
and undoubtedly important mouth structure by the 
book-lice and the bird-lice (and on the basis of other 
strong similarities) it seems certain that these two 
groups of insects have a common ancestry not very 
remote, and probably should be included in a single 
Fig. 140.—A wingless order. 
(Muchenl’argld.) 01 Sf> ' The 0rder Corrodentia as at present known con- 
tains about two hundred described species, scattered 
over the world. The largest species occur in Brazil, and have an ex¬ 
panse of wing of nearly an inch. Ceylon and the Hawaiian Islands are 
said by Sharp to be specially rich in species. 
The members of the order can be divided into two families as follows: 
Wings well developed; ocelli present (in addition to compound eyes).. .PsociD/E. 
Wings wanting or present as small scales or pads; ocelli absent_ Atropid^e. 
The winged Corrodentia or Psocidae (which may be called bark-lice to 
distinguish them from the wingless book-lice) are 
too rarely seen to be at all familiar. They may 
most commonly be found in small clusters on bark, 
each cluster or colony being covered over by fine 
silken threads spun from the mouth. The wings 
are held roof-shape over the back (Fig. 141), and 
the body and wings are usually pale smoky in 
general color. The small white eggs are laid on the 
surface of the bark in small patches, and in a cluster 
of bark-lice, individual in all stages, from very young to adult, may be seen. 
Banks gives the following key to the North American genera: 
Fig. 141.—A winged 
bark-louse. (Thirteen 
times natural size.) 
The techinal terms discoidal cell and posterior cell may be understood by reference to 
Fig. 142. 
1. Wings with scales and long hairs... Amphientomum. 
Wings without hairs and scales, hyaline. 2. 
2. Tarsi 3-jointed.3, 
Tarsi 2-jointed.4. 
