NO. 7 THE INSECT CRANIUM— SNODGRASS 57 



the labrum as other than the unseparated areas of the frous and 

 clypeus. If the cleavage lines were not present, it is doubtful if anyone 

 would think of making any other interpretation. In adult Diptera 

 the frontal and clypeal areas are usually distinctly separated, and the 

 clypeus is identified as such by the attachments on it of the dilator 

 muscles of the cibarial pump. 



As already noted, Cook (1944a) has shown the distribution of 

 muscle attachments on the frontoclypeal region in the head of the 

 mosquito larva ; the pharyngeal muscles are attached on the upper 

 part (fig. 12 D), the cibarial muscles (dlcb) laterally on the lower 

 part. In addition, however, are four groups of fibers, called by Cook 

 the "messorial" muscles, which vibrate the labral mouth brushes, and 

 which take their origin on the upper, presumably frontal, area of the 

 frontoclypeal region. Cook concludes that these "messorial" muscles 

 are probably specialized groups of cibarial fibers, but their functional 

 connection with the labrum and their frontal origin would suggest 

 that they are labral muscles, though admittedly it is difficult to identify 

 them with muscles of the labrum in other insects. 



The larva of Tipulidae may be taken as an example of the type of 

 dipterous larvae in which the head is retracted into the thorax but 

 still retains a relatively generalized structure. The dorsal surface of 

 the ovoid head (fig. 12 H) presents two deep, longitudinal sclerotic 

 grooves (gr, gr), between which the arms of the cleavage line {CL) 

 demark a narrow frontal area (/r) continuous anteriorly with a wider 

 clypeal area {dp) betwen their divergent distal parts, which end on 

 the clypeal margin just beyond the bases of the antennae. 



An examination of the head musculature of the tipulid larva shows 

 a generalized arrangement of the cibarial and precerebral stomodaeal 

 muscles (fig. 13 C), though the brain lies in the thorax. The cibarial 

 muscles {dlcb) arise on the clypeal region {dp), and the pharyngeal 

 muscles of the stomodaeum {pluncls) take their origin on the frontal 

 area (/r) between the cleavage lines. The two sets of muscles are 

 separated by the frontal ganglion connectives, which turn back along 

 the sides of the stomodaeum {FrCon) to the brain. On the frontal 

 area are attached also the single, median pair of labral muscles 

 {Ibrmcl). 



A quite different type of head structure is seen in the retracted head 

 of a tabanid larva (fig. 12 I). Here the head is slender and club- 

 shaped, thickened basally and tapering distally. On the dorsal surface 

 two longitudinal lines separate a long, narrow median area {Clp), 

 continuous distally with the labrum {Lm), from a pair of lateral 

 plates of the head that end proximally in tapering points on the dorsal 



