1915] The Head and Mouth-Parts of Thysanoptera 31 
extensive atrophy has taken place and those tentorial parts 
which still remain can only be interpreted by comparison 
with the more generalized conditions in the nymphs. 
On the cephalic aspect of the head of a nymph of Heliothrips 
(fig. 13) two sets of invaginations can be seen. One pair 
exists on the ventral margin of the front (i. a.) and the second 
pair (i. d.) a short-distance dorsad of these. Since the invagina- 
tions on the ventral margin of the front are close to the lateral 
edges of the wide clypeus, they actually occur on the lateral 
aspects of the head-capsule. This is especially true of the 
invaginations on the right side. Distinct chitinous arms extend 
between the invaginations on each side (a. a.). The invagi- 
nations, as described, are homologous with the two pairs of 
invaginations on the cephalic aspect of the head of Cicada. 
The two invaginations on the ventral margin of the front 
are believed to be the invaginations of the anterior arms of 
the tentorium (i. a.) while the invaginations dorsad of these 
are the invaginations of the dorsal arms of the tentorium 
(i. d.). The rods between the invaginations on each side are 
the anterior arms (a. a.). Chitinized thickenings (d. a.) 
extend dorso-caudad from each of the invaginations of the 
dorsal arms (i. d.) and these thickenings on reaching the caudal 
margin of the head-capsule turn caudad and run parallel 
with the caudal margin of the head (p. a.) to the distal ends 
of the caudal projections (c. a.). On comparison with Cicada 
the dorso-caudal extensions that project from the invaginations 
of the dorsal arms to the caudal margin of the head are homol- 
ogous with the dorsal arms of the tentorium (d. a.) while the 
chitinized thickenings along the lateral margins of the occipital 
forearm (p. a.) are considered as homologous with the posterior 
arms of the tentorium. In thrips the body of the tentorium 
is wanting, consequently the dorsal arms (d. a.) connect directly 
with the posterior arms (p. a.). The invaginations for the 
posterior arms (i. p.) were not identified in any of the thrips 
examined. Ina few cases it seemed as though the invaginations 
were present near the ventral end of the posterior arms (p. a.) 
where the thickenings (x.) on the ventral margin of the head- 
capsule came in contact with the posterior arms. Thus far 
the tentorium of a nymph of the Terebrantia can be homologized 
with the tentorium of Cicada. 
