THE THORAX AND ITS APPENDAGES. 59 



part under discussion. We hence see that not only the first abdomi- 

 nal tergum but the entire segment has undergone transposition, 

 though the ventral part has disappeared in all the higher families. 

 This transferred part has been named both the median segment and 

 the propodeum, by writers who recognize it as belonging to the abdo- 

 men and not to the thorax. 



The names current among systematists for the back plates of 

 Hymenoptera afford an excellent example of the errors that ento- 

 mologists may be led into through an ignorance of the comparative 

 anatomy of insects. They recognize the protergum as such and then, 

 loiowing that there are yet two segments to be accounted for, they 

 call the mesoscutum the " mesonotum," the mesoscutellum the 

 " scutellum," the metatergum the " postscutellum " (being unaware 

 that the true postscutellum is deeply concealed within the thorax), 

 while the first abdominal tergum is called the metathorax. Such 

 a nomenclature assigns both pairs of wings to the mesothorax. Too 

 many systematists working in only one order of insects do not care 

 whether their names are applied with anatomical consistency or not. 



2. THE WINGS AND THEIR AKTICULATION. 



In the study of insects the wings always form a most interesting 

 subject because by them insects are endowed with that most coveted 

 function — the power of flight. It has already been stated that the 

 wings are not primary embryonic appendages, but are secondary out- 

 growths of the body wall from the second and third thoracic seg- 

 ments. Therefore it is most probable that the early progenitors of 

 insects were wingless, yet for millions of years back in geological time 

 they have possessed these organs in a pretty well developed condition. 

 . Nearly all of the insect orders have some characteristic modifica- 

 tion of the wingf-veins and their branches. None of them, however, 

 departs nearly so far from the normal type as do the Hymenoptera, 

 even the lowest members of this group possessing a highly specialized 

 venation. Before beginning a study of the Hymenopteran series 

 which leads up to the bee the student should first turn back to figure 

 C (p. 22) and again familiarize himself with the generalized condi- 

 tion of the veins and the articular elements of the wing. By com- 

 paring, now, with this diagram the basal parts of the wing of a 

 sawfly {Itycorsia discolor, fig. 26 A) it will be easy to identify the 

 parts of the latter. Vein C has two little nodules {C, C) cut off from 

 its basal end which lie free in the axillary membrane. Yein Sc articu- 

 lates by an enlarged and contorted base {8c) with the first axillary 

 {lAx), while vein R is continuous with the second [2 Ax). The next 

 two veins that come to the base and unite with each other are appar- 

 ently not the media and cubitus but the first and third anals {lA and 



