522 PROCEEDIl^GS OF THE NATIONAL MVHETJM. vol. xxxvi. 



There is evidently a lack of harmony here unless it be that the first 

 maxillae of the Chilopoda correspond with the superlinguse of the 

 Insecta, the second maxilla3 with the maxillae, and the poison claws 

 with the labium. In this case we could regard the microthorax of 

 insects as the maxilliped segment of the Chilopods, which, from 

 superficial appearances, would not seem impossible. 



The sclerites of the microthorax are well known. They have been 

 studied extensively by Verhoetf (1902) and occur in nearly all the 

 orders of insects. They are specially w^ell developed in the Odonata 

 (5, 6, 7, 8, 9, 12, Ml and 1 mi, '2 ml, 3 mi, 4 mi), in the Orthoptera 

 (24, 25, 36, 37, 45) , and in the Euplexoptera (93) , but occur in a more 

 reduced condition in many of the other orders, such as the Coleoptera 

 (95, Mi) and the Diptera (174, mi, mi). In the Orthoptera and 

 Euplexoptera they often form an almost complete segment present- 

 ing tergal, pleural, and sternal plates. Verhoeff has gone so far as 

 to identify all the pleurites of a thoracic segment in the microthorax, 

 but undoubtedly this is establishing homologies on a too imaginative 

 basis. Comstock and Kochi (1902) regard the gular sclerites of the 

 head as the microthoracic sternum, and in some of the Euplexoptera 

 (93) the microthoracic sternites are so large and so associated with 

 the head as to suggest the gular sclerite of the Coleoptera. 



We may conclude that there is no reason for regarding the micro- 

 thorax as anything more than the neck segment whose sclerites are 

 reduced to the small neck sclerites and the gular plate when the lat- 

 ter is present, whose ganglion has fused with the last head ganglion, 

 and whose fused appendages become attached to the head in most 

 cases and constitute the labium. It should not be included, in reck- 

 onings of the number of segments forming the head, as one of the head 

 segments. (See note on page 595.) 



IV. THE THORAX. 



In a former paper (1908) the writer gave a brief account of the 

 structure of the insect thorax. This description can now be amplified 

 by illustrations taken from all the principal orders. Vov convenience 

 the subject will be divided under three heads, namely, (1) the ter- 

 gum, (2) the pleurum and coxa, and (3) the sternum. 



1. The Tergum. 



The word tergum is here used to designate all the chitinized parts 

 of the dorsum of any segment. It is generally used interchangeably 

 with the term " notum," but where the tergum consists of two plates 

 the latter name, notinn, will be restricted in this paper to the first 

 or wing-bearing sclerite, and the term pGstnofvm- or pseudonotiim 

 (Verhoeff, 1903) applied to the posterior or post-alary plate. The 

 notum is the plate which, by diversities of its surface topography, 



