50 



AN INTRODUCTION TO ENTOMOLOGY 



chitinized plate; this sclerite is designated the notum. The term 

 notum is also applied to the tergal plate of the prothorax and to that 

 of each abdominal segment. The three thoracic nota are designated 

 as the pronotum, the mesonotum, and the metanotmn respectively. 



The notum of a wing-bearing segment is the part that bears the 

 wings of that segment, even when the tergum contains more than one 

 sclerite. Each wing is attached to two processes of the notum, the 

 anterior notal process (Fig. 61, a n p) and the posterior notal process 

 (Fig. 61, p n p); and the posterior angles of the notum are produced 

 into the axillary cords, which form the posterior margins of the basal 

 membranes of the wings (Fig. 61, Ax C). 



The postnotum or postscutellum. In the wing-bearing segments of 

 most adult insects the tergum consists of two principal sclerites ; the 

 notum already described, and behind this a narrower, transverse 

 sclerite which is commonly known as the postscutellum, and to which 

 Snodgrass has applied the term postnotum (Fig. 61, P N}. 



The divisions of the notum. In most specialized insects the notum 

 of each wing-bearing segment is more or less distinctly divided by 

 transverse lines or sutures into three parts; these are known as the 

 prescutum (Fig. 61, Psc), the scutum (Fig. 61, Set), and the scutellum 

 (Fig. 61, Scl). 



It has been commonly held, since the 

 days of Audouin, that the tergum of each 

 thoracic segment is composed typically of four 

 sclerites, the prescutum, scutum, scutellum, 

 and postscutellum. But the investigations of 

 Snodgrass indicate that in its more genera- 

 lized form the tergum contains a single 

 sclerite, the notum; that the postscutellum 

 or postnotum is a secondary tergal chitini- 

 zation in the dorsal membrane behind the 

 notum, in more specialized insects; and that 

 the separation of the notum into three parts, 

 the prescutum, scutum, and scutellum, is a 

 still later specialization that has arisen 

 independently in difterent orders, and does 

 not indicate a division into homologous 

 parts in all orders where it exists. 



The patagia. In many of the more 



specialized Lepidoptera the pronotum Fig . 6l . -Diagram of a generalized 

 IS produced on each side into a flat thoracic segment (From Snod- 



lobe, which in some cases is even con- ^ 



stricted at the base so as to become a stalked plate, these lobes are 



the patagia. 



hi 



