NO. I INSECT THORAX — SNODGRASS 63 



with the distal end of the third axillary. A fourth axillary is some- 

 times present (Orthoptera, Hemiptera, Hymenoptera) , but it is 

 always a small sclerite of the dorsal surface of the wing base, inter- 

 vening between the inner end of the third axillary and the edge of 

 the tergum (fig. 24 B, 4AX). It is perhaps a detached piece of the 

 posterior notal wing process. Lying in the space between the second 

 axillary, the distal part of the third axillary, and the bases of the 

 median and cubital veins, there are usually two sclerites of less 

 definite form which may be termed the median plates of the wing base 

 (fig. 27, in, in'). These sclerites serve to connect the median field of 

 the wing with the true axillaries. 



THE WING MUSCLES AND THE MECHANISM OF WING MOTION 



The mechanism of insect flight was carefully studied by entomol- 

 ogists in the early part of the last century. Noteworthy papers pub- 

 lished on the subject at that time are those of Chabrier (i82o-'22), 

 Jurine (1820), and Straus-Diirckheim (1828). Chabrier 's work, the 

 first of any importance on the wing structure of insects, contains 

 detailed descriptions of the skeletal anatomy of the thorax, the struc- 

 ture and articulation of the wings, and the muscles and mechanism 

 of flight — an extensive piece of original investigation, remarkable 

 for its accuracy and for the understanding shown by the author for 

 his subject. Following a general account, there is given detailed 

 descriptions of the entire wing mechanism in Coleoptera (Mclolon- 

 tha), Odonata (Aeschna), and Ilymenoptera (Bonibns). Jurine's 

 paper, which describes the wing mechanism of Hymenoptera, ap- 

 peared after the first section of Chabrier's work was published. The 

 great work of Straus-Durckheim (or Straus-Diirckheim, as his sig- 

 nature appears beneath the dedication to Cuvier), published under 

 the title, " Considerationes generales sur I'anatomie comparee des 

 animaux articules," is devoted principally to a study of Melolontha 

 vulgaris. It is one of the finest monographs ever written on insect 

 anatomy, and is accompanied by figures unsurpassed in clarity of 

 detail by any methods of modern illustration. The thorax, the wing 

 muscles, and the mechanism of flight are given full attention. These 

 early works, unfortunately, are somewhat difficult for present-day 

 students to read, because their authors used mostly individual sys- 

 tems of nomenclature, none of which has been closely followed by 

 later entomologists. 



The modern study of the wing mechanism begins with Lendenfeld 

 (1881) and Amans (1883, '84, 1884, 1885), though again the nomen- 



