776 AN INTRODUCTION TO ENTOMOLOG Y 



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

 is termed the frontal suture. Frequently the frontal suture extends 

 down on each side to near the mouth (Fig. 991,/. s). 



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

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

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

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

 opening through which the adult escapes; afterwards the ptilinum is 



withdrawn into the 



head. If a specimen 



is captured soon 



^r->, after its emergence 



/ Y from the pupar ium , 



^ ^^;?=:Ca^ // there may be seen 



/y'ZT~Sc^--.L "\J: // insteadofthe 



// /^^---A >^^^^ f"^ frontal suture the 



/ \ /^'^77,: =c^^^p^p^^-iNi I /fWr 7 \ bladder-lilve ptili- 



W^^yi ^ ^iJT^ \ ^/~Vy ^A) ^ / J^ numprojecting 

 r^^^^ ^^^^'^^\ rxh^v /— — ( from the head, im- 

 \~^^^^^^ . ^J^\. \^-^^^<^^^_^ mediately above 



^ ^^'^^>^L^.--^^\.'tiw^^^^^^^^'^^^ ^^® antennas. 



\j_,^<-^~( ^v. - Aes^ V^*" M ^ -^^^^ thorax and 



^"""^ X^x'' Nn X"'^''^<1^ a ^'^-^ appendages. — 



\^J^'^" — '^x.'^'' v^""^^^^^ '^^^ thoracic region 



^J \^ ^ of the body con- 



\cp sists chiefly of the 



mesothorax, both 



T X 1 ^ cu t T, 1 T -e the prothorax and 

 Fig. 992.— Lateral aspect of thorax of P achy r Jnna f err u- , rne^fa-thnrax bp 

 ginea. (After Young). The thoracic segments to P® metatliorax De- 

 which the sclerites belong are indicated by the num- mg greatly reduced 

 bers I, 2, and 3. A, A, A, first abdominal segment; in size. The thorax 

 ae?n, anepimerum, the upper part of the epimerum; q£ ^ crane-fly (Fig 

 aes, anepisternum, the upper part of the episternum; , .,, ^ " 

 ex, coxa; em, epimerum of the metathorax; h, halter; 992;, 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^g more general- 

 scutum; si, scutellum; sp, spiracle; tr, trachantin. .^^^ 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 ('2 1) who studied 

 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 



