NO. I INSECT THORAX — SNODGRASS 55 



ception of their nature, and which when logically carried out does 

 not lead to conflicting results. 



In the mesothorax of Tipulidae, the tergal area between the pre- 

 scutum (fig. 24, D, Psc) and the scutellum (Scl) is partially divided 

 into several regions by pairs of oblique lateral sutures. The sutures 

 constituting the first pair (par) extend inward and posteriorly a short 

 distance from the lateral parts of the prescuto-scutal suture (ps); 

 those of the second pair (x) arise before the bases of the wings and 

 converge posteriorly and medially; those of the third pair (y) go 

 from the region of the wing base posteriorly and medially toward the 

 median field of the scutellum. Each suture is the external line of a 

 corresponding internal ridge, and the spaces between the ridges are 

 areas of muscle attachments. The anterior ends of the dorsal longi- 

 tudinal muscles are attached on the median area (a) between the 

 first pair of ridges (par). The tergo-sternal muscles (fig. 34 D, C) 

 are attached laterally on the spaces (c) behind these ridges ; the tergo- 

 meron muscles (fig. 34 D, /) are attached on the lateral areas (j) 

 between the second and third ridges ; and the tergo-phragma mus- 

 cles (fig. 34 D, 5) are attached behind the third ridges. These lateral 

 tergal muscles in the Diptera all act as elevators of the wings. 



The first pair of lateral tergal ridges in the Tipulidae (fig. 24 D, 

 par) are clearly the homologues of the so-called parapsides, or parap- 

 sidal ridges, of other insects (E, F, I, par), which are usually re- 

 garded as defining a median triangular posterior extension of the 

 prescutum (the protcrgite of Berlese). It is here proposed, however, 

 that the parapsides are merely secondary ridges of the scutum, and 

 that the median area between them is, therefore, a part of the scutum. 

 The ridges are correlated with the extension of the anterior ends of 

 the dorsal longitudinal muscles on the tergum : as these muscles en- 

 large, their anterior bases encroach first upon the prescutum, then 

 upon the scutum (fig. 29, A, A), and they may finally come to occupy 

 almost the entire median field of the latter (fig. 34 D, A, A). In 

 the higher Diptera, in which the median part of the prescutum and 

 scutum are not distinct (fig. 24 E), the anterior ends of the parap- 

 sidal sutures (par) are obsolete; their posterior ends unite with the 

 next pair of lateral sutures (x). In Cynipidae, the parapsidal sutures 

 (F, par) extend backward to the scuto-scutellar suture and cut the 

 scutum lengthwise into three areas (c, a, c). In the mesothorax of 

 Cicadidae and Tenthredinidae, the parapsidal sutures define an an- 

 terior median triangular area of the scutum; in the former (fig. 29), 

 the prescutum is not distinct, but in the latter family it is present 

 though very narrow (fig. 24 I, Psc). In the mesothorax of Lepidop- 



