STRUCTURE OF THE THORAX 05 



THE THORAX. 



The terminology of the parts of the thorax of mosquitoes is very confused and 

 shows that its structure has not been generally understood. We, on this account, 

 requested Mr, E. E. Snodgrass, formerly of the Bureau of Entomology, U. S. 

 Department of Agriculture, who has made a study of the homologies of the 

 thorax of insects of different orders, to prepare the following study, with figure, 

 of the thorax of a mosquito, Psorophora dliata (see plate III). 



The thorax of the mosquito is conspicuously wedge-shaped, the base being 

 uppermost and forming the large rounded dorsum of the thorax. The triangular 

 sides form the pleura, while the apex is ventral and carries the legs. The wings 

 (w^) are inserted on the upper parts of the sides much behind the middle, so 

 that the greater part of the thorax is in front of them, but the balance of the 

 entire body is preserved by the long, slender abdomen extending posteriorly. 

 The second wings are represented by the halteres (w^) which, of course, take no 

 part in the mechanism of flight. Consequently the great muscles of the meso- 

 thorax alone afford the motive power of flying. The active function of the pro- 

 thorax and the metathorax is the support of the front and hind legs. Hence, 

 both of these segments consist of little more than anterior and posterior collars 

 against the great mesothoracic mass. The consolidation of the three segments 

 is so complete that many entomologists find it difficult to define their limits with 

 certainty. 



Two conspicuous points on each side of the thorax are the spiracles {ISp and 

 2Sp). Each is a large black-rimmed aperture situated in a membranous area, 

 one near the middle of the anterior half of the side, the other somewhat behind 

 the middle of the posterior half. Since, in most adult insects, these thoracic 

 spiracles are situated in the intersegmental membranes, it is a difficult matter to 

 decide to which segment each belongs. The second is nearly always regarded 

 as metathoracic, but entomologists are divided in opinion as to whether the first 

 is mesothoracic or prothoracic. Amongst morphologists, however, the first view 

 seems to have the greater support. However, this question is of little importance 

 for the more practical needs of the systematist, who may avoid the difficulty by 

 referring to them simply as the first and second thoracic spiracles. 



The ProtJiorax. — The lateral walls of this segment consist of three plates 

 (Ti, EpSi, and Epm-y). Closely associated with them is a large cervical sclerite 

 {mi) lying on each side of the neck and forming a support for the head an- 

 teriorly. The most conspicuous of the prothoracic plates is the prominent oval 

 lobe (Ti) beneath the projecting anterior end of the mesotergum. This very 

 probably is a lateral part of the protergum, for it is connected by a median 

 narrow transverse bridge in front of the mesotergum with the corresponding part 

 of the opposite side. These two lobes are called " patagia " by Christophers, a 

 name properly belonging to the prothoracic tergal appendages of Lepidoptera, 

 but of doubtful propriety when applied to similar parts in the mosquito, because 

 there can be no homology between them. Connected with the lower end of the 

 protergal lobe is a stalk-like piece which expands ventrally into a larger plate 



