STRUCTURE OF THE MALLOPHAGA. 161 



{a) With sharply marked-off clypeus and 



shallow orbital sinus ... Lamohothrium. 



(d) With only wavy head-margins, and 



long lateral lobes on the labrum... Physostomum. 

 2. Mesothorax present. 



a. Mesothorax large, sharply marked-off, 



head three-sided, antennae concealed Trinotum. 



b. Mesothorax small, only indicated. 



(a) Orbital bay deep, antennae mostly 



elongated and visible ... Colpocephaluin. 



{b) Orbital bay very shallow or obso- 

 lete, antennae concealed ... Me7iopon. 



Crosse's researches have been largely on a Liotheid found on 

 a pelican from Chili, closely related to Menopon^ and forming the 

 type of a new genus and species, Tetrophthalmiis chilensis. The 

 male is 4 — 4|mm long, the female slightly less. He also contri- 

 butes important emendations of our knowledge of the other 

 species. 



Head. — In Tetrophthalmus the head is somewhat constricted, 

 is broader than long, slightly convex above, concave below, and 

 somewhat uniform, the occipital angles being rounded off. The 

 hinder limit of the clypeus shows on each side a notch, about a 

 third from the front of the head ; two dark spots are seen on each 

 side of the head, the larger one near the notch, the other behind 

 it and outwards. The antennae lie concealed in a lateral cavity of 

 the under side of the head (as in Lcemohothriiim^ PL XX., Fig. i, at). 

 Two eyes, whose pigment is seen from above, lie on each side below 

 and behind the antennal cavity. Hairs are distributed over the 

 head, along the borders, and on its ventral and dorsal surfaces. 

 On the under-side of the head is the funnel-shaped mouth-open- 

 ing, surrounded by the mandibulate mouth-parts. Crosse de- 

 scribes the mouth-parts of Mallophaga in detail, as previous 

 writers err greatly regarding them. 



Labrum (oberlippe). — This is not, as in other insects, inserted 

 on the anterior border of the head, but in all Mallophaga it is on 

 the under side of the head. In all Liotheid^ it is similarly 

 formed (Fig. i, /^.), being a thin transverse arched swelling, with 



