112 Book-lice and Bark-lice; Biting Bird-lice 



there is a peculiar little chitinized structure, which may be called the 

 ccsophafceal scierite (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 

 j[\ I to the book-lice alone. But, in a study of the mouth 

 structure of the biting bird-lice (Malloj)haga), I found 

 an almost identical oesophageal scierite in thirteen out 

 of the twenty-two genera of the Malloohaga. On 

 the basis of this common ])ossession 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 

 ,.^x remote, and probably should be included in a single 



Fig. 140. — A wingless order, 

 book-louse, Airopos sp. 'pj^g g^der Corrodentia as at present known con- 

 (Much enlarged.) . , , , , 1 ■, , • , 



tarns 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). . .Psocm,E. 

 Wings wanting or present as small scales or pads; ocelli absent ATR0PID.aj. 



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 i | 

 the body and wings are usually pale smoky in p,j, ,^j _^ winged 

 general color. The small white eggs are laid on the bark-louse. (Thirteen 

 surface of the bark in small patches, and in a cluster '■''^" ■'^'"'■='' ^'^'^•^ 

 of bark -lice, individual in all stages, from very young to aiiult, may be seen. 



Banks gives the following key to the North American genera: 



The lechinal terms discoidal cell and posterior cell may be understood by reference to 

 Fig. 142- 



1. Wings with scales and long hairs Amphientomtjm. 



Wings without hairs and scales, hyaline 2. 



2. Tarsi 3-jointcd 3. 



Tarsi 2-jointed 4- 



