6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



improbable, however, that these three pieces are primary segments 

 of the insect's thorax and not, as Kiinckel and Lowne think, parts 

 of the mesothorax. I will not here pursue this question further, 

 and provisionally accept the ideas of Kiinckel and Lowne in spite 

 of my doubts as to their correctness. 



3. The Skeleton of the Thorax, so far as it Relates to Flight 



The part of the thorax most important for flight is the meso- 

 thorax. In its skeleton we may distinguish exoskeletal and endo- 

 skeletal parts. 



exoskeletal parts 

 These are the mesonotum, the mesopleura and the mesosternum. 



TJie Mesonotum 



The mesonotum consists of four parts, the pr^scutum, scutum, 

 scutellum and postscutellum. 



The prcesctitum (pi. i. fig. i ; pi. 2, figs. 3, 4; pi. 8, fig. 23 ; p), to- 

 gether with the scutum, scutellum, parapleurum and mesosternum, 

 forms a group of the largest and strongest chitinous plates of the 

 thorax. Beginning in the region of the cervical opening, the prse- 

 scutum rises vertically upward, assumes near the limit between the 

 first and second third of its length a horizontal position, and at the 

 same time broadens out. From the angle between these two sur- 

 faces backward it is covered with hair. The hairs are directed 

 backward and increase in length toward the scutellum. They are of 

 two kinds: small, fine hairs and coarse bristles. The latter are 

 arranged in irregular, longitudinal, and transverse rows. On the 

 prajscutum six longitudinal and three transverse rows of bristles 

 can usually be distinguished. They fall out when the thorax is 

 treated with liquor potassae, and their places of attachment then 

 appear as circular holes. The hairy covering o^f the back appears to 

 be especially liable to injury. The small hairs also are often missing 

 in many places, their points of attachment appearing as small dots. 

 Two shallow, longitudinal furrows divide the pra^scutum into three 

 zones. These are continued on the scutum but are absent on the 

 scutellum. Ventrally, the pr?escutum is separated from the para- 

 pleurum by the pleural cleft. Laterally, toward the scutum, tri- 

 angular areas, feebly chitinized and clear in appearance (pi. 2, fig. 

 3), border on the praescutum. 



