MORPHOLOGY 



745 



metatarsus) ; some authorities have caused much confusion by caUing the 

 first tarsal joint the metatarsus, and the real second tarsal joint the first 

 tarsal joint, and so on. The last tarsal joint is terminated by claws or ungues, 

 between which other appendages called empodia and pulvilli are to be found, 

 as will be explained later. 



Typically, the mesothorax and the metathbrax should each carry a pair of 

 wings. These are transparent and strengthened by nervures, ribs, or veins, 

 which are chitinous canals containing blood-spaces, nerves, and tracheae. 

 The areas between the nervures are called cells. The wings, however, become 

 much modified in the different orders, and may be entirely absent. The 

 arrangement of the nervures in the wing is called the ' venation,' and has 

 been restudied by Comstock and Needham, who find that the primitive type 

 is composed of two main tracheal branches, an anterior and a posterior. 

 The anterior breaks up at the base of the wing into four longitudinal branches 

 — the costa, subcosta, radius, and media — while the posterior has also four 

 branches, of which the first is called the cubitus, and maybe subdivided into 

 two ; and the other three are simple, and are called the anal veins — first, second, 

 and third. The costal vein is unbranched, and runs along the anterior margin 

 of the wing. The subcostal vein typically divides into two branches, the 

 radial vein into five branches, and the median into four. 



This primitive type is altered by atrophy or coalescence, leading to the 

 reduction of the veins. The latter may take place from the base towards the 

 tip, or from the tip towards the base of the wing, called outward and inward 

 coalescence respectively. The wing cells may be named as follows: — 



1. Costal Cell, between the Costa and the Subcosta. 



2. Mediastinal, between the Subcosta and the Radius. 



3. Marginal, between Radius i and Radius 2. 



4. First Submarginal, between Radius 2 and Radius 3. 



5. Second Submarginal, between Radius 3 and Radius 4*. 



6. Third Submarginal, between Radius 4 and Radius 5. 



7. First Posterior, between Radius 5 and Media i. 



8. Second Posterior, between Media i and Media 2. 



9. Third Posterior, between Media 2 and Media 3. 



10. Fourth Posterior, between Media 3 and Media 4. 



11. Fifth Posterior, between Media 4 and Media 5. 



12. Sixth Posterior, between Media 5 and Cubitus i. 



13. Seventh Posterior, between Cubitus i and Cubitus 2. 



14. First Anal, between Cubitus 2 and" Anal i. 



15. Second Anal, between Anal i and Anal 2. 



16. Axillary, between Anal 2 and Anal 3. 



17. Spurious Cell, behind Anal 3. 



But all these cells are not present in any one given type of wing, owing to 

 coalescence of the nervures, and hence the arrangement of the cells is different. 

 Besides this, the venation is complicated by the presence of transverse veins, 

 as will be explained later; moreover, wings may be absent, as in the lice. 



The abdomen usually consists of ten somites, without appendages, com- 

 posed of dorsal and ventral plates connected together and to preceding and 

 succeeding segments by soft membranes. The posterior segments are often 

 modified with reference to reproduction, possessing claspers in the male, and 

 ovipositors in the female. The anal opening is on the last abdominal segment , 

 and the reproductive aperture on the penultimate segment. 



Internal Anatomy, — The mouth lies between the labrum above and the 

 labium below, and leads into an oral cavity. 



The salivary glands in most of the genera which we have to consider are well 

 developed, and are, as a rule, not connected with the mouth, but open into 

 a common duct which communicates with a groove or canal on the hypo- 

 pharynx, and so opens near the tip of the proboscis. 



From the mouth a pharynx leads through an cEsophagus, with a dilatation 

 called the crop, into a proventriculus or masticatory stomach, which latter 

 communicates with the mesenteron or chylific ventricle, whose juncture with 



