NO. 7 THE INSECT CRANIUM — SNODGRASS 39 



surface. Laterad of the ends of these plates are two small elongate 

 oval sclerites, but the rest of the basal part of the head is weakly 

 sclerotized. When the head capsule is opened it is seen to contain three 

 long, strongly sclerotic rods, two of which are lateral, and one median. 

 The lateral rods arise at the base of the labrum and are attached pos- 

 teriorly by their apodemal inflections to the posterior oval sclerites 

 of the head surface. These rods evidently are tentorial arms. The 

 median rod is the sclerotic floor of a tube extending the full length 

 of the head (fig. 13 E, CbP), opening posteriorly into the oesophagus 

 {Oe). This rod is clearly the floor (sitophore) of the greatly elongate, 

 tubular cibarium; on the invaginated roof of the tube are inserted 

 a series of paired muscles (dlcb) that arise on the median area of the 

 dorsal head wall (fig. 12 I, Clp), which is thus seen to be the clypeus. 

 In addition to the paired dilator muscles there is present also a median 

 cibarial muscle (fig. 13 E, mcbincl) inserted anteriorly by a long 

 tendon. Confirmatory evidence that the sucking tube of the tabanid 

 larva is the cibarium is seen in the fact that on a descending arm from 

 the posterior end of its floor are attached the huge muscles of the 

 unusually large salivary pump {SIP). The salivary pump muscles 

 are always attached on the floor of the sucking pump in Diptera 

 having a salivary pump, since these muscles primarily are hypo- 

 pharyngeal and the floor of the pump is the sitophore of the hypo- 

 pharynx in generalized insects. 



At ecdysis the head of the tabanid larva shows none of the usual 

 exuvial clefts. The soft posterior part of the head is torn off, while 

 the sclerotic parts remain intact (fig. 12 J), with the tentorial arms 

 {AT) and the sclerotic floor of the sucking pump {CbP) projecting 

 from the posterior opening. It is to be noted that this exuvial remnant 

 of the tabanid larval head is little more than the so-called "pharyngeal" 

 skeleton of a cyclorrhaphous larva, which correctly is the combined 

 clypeus and cibarial pump, represented in the adult fly by the com- 

 posite structure commonly known as the "fulcrum." 



Hymenoptcra. — There is not much information available on the 

 manner of ecdysis in Hymenoptera, but a few observations make it 

 appear that the splitting of the head cuticle follows the usual lines 

 in the Tenthredinoidea, and that in the higher Hymenoptera it is 

 limited to a cleft down the middle of the face. 



The head of a tenthredinid larva, as seen in Neodipnon (fig. 14 A), 

 presents a prominent forked line of cleavage {CL), the widely 

 divergent arms of which enclose a large quadrate facial region extend- 

 ing to the clypeal margin. The muscles attached on this region are 

 shown by Parker (1934) in Psendoclavellaria amerinae (L.), and by 



