776 



AN INTRODUCTION TO ENTOMOLOGY 



separating the frontal lunule from that part of the head iihove it ; this 

 is termed the frontal suture. Frequently the frontal suture extends 

 down on each side to near the mouth (Fig. ggi,f.s). 



The ptUinmn is a large bladder-like organ which exists in those 

 flies that have a frontal suture. The ptilintmi is pushed out through 

 this suture when the adult is about to emerge from the puparium. In 

 this way the head end of the pupariimi is forced off, making a large 

 opening through which the adult escapes; afterwards theptilinumis 



withdrawn into the 

 head. If a specimen 

 is captured soon 

 after its emergence 

 from the puparium, 

 there may be seen 

 insteadofthe 

 frontal suture the 

 bladder-like ptili- 

 numprojecting 

 from the head, im- 

 mediately above 

 the antenncB. 



The thorax and 

 its appendages. — 

 The thoracic region 

 of the body con- 

 sists chiefly of the 

 mesothorax, both 

 the prothorax and 

 the metathorax be- 

 ing greatly reduced 

 in size. The thorax 



Fig. 992. — Lateral aspect of thorax of Pachyrhina ferru- 

 ginea. (After Young). The thoracic segments to 

 which the sclerites belong are indicated by the num- 

 bers I, 2, and 3. A, A, A, first abdominal segment; 



aem, anepimerum, the upper part of the epimerum; „r „ crane-flv fFip" 

 aes, anepisternum, the upper part of the episternum; s .,, -^ ; °' 



ex, coxa; em, epimerum of the metathorax; h, halter; 992 J, Will serve to 



kem, katepimerum, the upper part of the epimerum; illustrate the struc- 



kes, katepisternum, the lower part of the episternum; ture of this part in 



me, meron; psc, prescutum; psl, postscutellum ; sc, j-j^g more P'cneral- 



scutum; si, scutellum; sp, spiracle; tr, trachantin. . h P ^ 



ized members of 



this order, and will 

 also serve as a type 

 with which to compare the thorax of the more specialized forms. 



There are differences of opinion among writers on this order re- 

 garding the homologies of certain thoracic sclerities. The most ex- 

 tended investigation of this subject is that of Young ('21) whostudied 

 and figured the thorax of representatives of fifty-five of the fifty-nine 

 families found in our fauna. I have adopted this writer's conclusions 

 regarding the homologies of the sclerites in question. 



The most distinctive feature of the wings of the Diptera is the fact 

 that only the first pair are developed as organs of flight ; the second 



