NO. 2 ENCYCLOPEDIA OF INSECT ANATOMY — SNODGRASS 29 



segment, and suggests that the neck mechanism has been secondarily 

 developed in the different insect orders. 



Gula : The term is derived from the Latin word for gullet, wind- 

 pipe, and neck, and in vertebrate anatomy it is used for the upper 

 part of the ventral side of the neck next to the chin. In insect 

 anatomy the gula refers to a ventral plate of the neck behind the 

 base of the labium. It is commonly continuous with the postocciput 

 of the cranium and may become united with the submentum of the 

 labium behind the posterior tentorial pits. Since the anterior part 

 of the neck is probably a membranous posterior part of the labial 

 segment, both the postocciput and the gula appear to belong to the 

 labial segment. The cervical sclerites lie behind the gula. A review 

 of the literature is given by DuPorte (1962). 



Thorax: The term is derived from the Greek word thorax, a 

 breastplate of ancient Grecian armor; in anatomy it refers to the 

 part of the human body covered by a breastplate. The thorax of in- 

 sects is the locomotor section of the body between the head and the 

 abdomen. It consists of three segments, the prothorax, mesothorax, 

 and metathorax, as a result of the reduction of the number of walk- 

 ing legs to three pairs. Once established as the locomotor center, the 

 thorax also became the site of wing development in the winged in- 

 sects. Wings, however, are present only on the mesothorax and the 

 metathorax, but either one of these pairs may be transformed into 

 nonflight organs. 



The thoracic wall of pterygote insects is necessarily well sclero- 

 tized and the presence of both legs and wings differentiates the cir- 

 cumference of the segments into tergal, pleural, and sternal. The 

 tergal plates are termed nota (Greek) in order to combine properly 

 with the Greek prefixes pro-, ineso-, and mcta-. The notum of the 

 prothorax is relatively simple because of the absence of wings on 

 this segment. In the winged segments, however, the notum becomes 

 the essential lever for the wing movement since its lateral margins 

 must vibrate up and down to give the vertical movement of the 

 wings in flight. The notal movements result from an alternating 

 longitudinal upward curvature and flattening of the notum produced 

 by constriction of longitudinal and vertical muscles. In adaptation to 

 its function, the wing-bearing notum must be properly flexible. It 

 is typically divided by a V-shaped ridge- forming groove in the pos- 

 terior part (the apex of the V is forward) ; this apparently controls 

 the bending of the notum so that the principal lateral movements 



