XI 



PHYLUM ARTHROPODA 



631 



Diptera and Hymenoptera. Again, the humming sounds charac- 

 teristic of many of the last-named order are produced partly by 

 the vibrations of the wings in flight, partly by the vibration of 

 leaf-like appendages in the tracheae set in motion by strong ex- 

 piratory currents of air. The loud shrill note of the Cicada is 

 produced by the rapidly recurring contractions of the fibres of a 

 muscle inserted into a stiff chitinous membrane, the result being 

 a series of crackling sounds, which follow one another so rapidly 

 as to give rise to a continuous note. 



Reproductive organs. The sexes are always separate in 

 Insects, as in Arthropoda in general ; and the males and females 

 are very commonly distinguishable from one another by various 

 modifications of 

 form and of color- 

 ation. There are 

 two ovaries, each 

 of which consists 

 of a greater or 



smaller number of \|p^^ ^f''y 

 narrow tubes or T/% */A/V d 



ovarioles ; in each oci % 'j/t 



of these the ova ^w/ <*< 



are arranged in a 

 single row the 

 early stages i n 

 their formation 

 being situated at 

 the anterior end, 

 the more mature 



OVa towards the FIG. 536.^4, female and B, male sexual apparatus of the Honey- 

 n n e f a " ^ ee ' ' accessor y glands ; de, common ejaculatory duct ; 



ffd, poison-glands ; gb, poison-vesicle ; ks, bulb of the stinging 

 F a P h apparatus ; md, rectum, twisted back and cut off ; nva, acces- 



sory sac of the vagina (bursa copulatrix) ; od, oviduct ; ov, 



OI OVanan ovary ; p, penis ; rs, receptaculum seminis ; sd, colleterial 



gland ; t, testes ; va, vagina ; vd, sperm-ducts. (From Lang's 

 tubes Opens into a Comparative Anatomy.) 



lateral oviduct, 



and the two -lateral oviducts, right and left (Fig. 536, A, od.), in most 

 cases unite behind to form a median oviduct or vagina (va.), which 

 opens towards the posterior end of the abdomen. Connected with 

 this median oviduct, or opening close to it, are receptacula seminis 

 (rs.) and colleterial or cement-glands (sd.). Sometimes there is a 

 copulatory sac, or bursa copulatrix (nva.). In the male the paired 

 testes (B, t.) vary greatly in form : sometimes each is a long, narrow 

 tube ; sometimes several such tubes combine to form the testis ; or 

 it may be of more compact rounded form and entire or lobed. 

 Each testis has a slender duct or vas defer ens (B, vd), the two 

 vasa deferentia uniting to form a median ejaculatory duct. A 

 vesicula seminalis is .appended to each vas deferens or to the 



a 9 



