44 2 DIPTERA CHAP. 



part of the antenna beyond the scape is called the " flagellum " ; 

 an appendage of the flagellum is called " arista " when bristle- 

 like, when thicker " style." In the basal joint of the antenna 

 there is a complex nervous structure known as Johnston's organ. 

 It is specially well developed in Culex and CMronomus, and is 

 larger in the male than it is in the female. Child has found 

 something of the kind present in all the Diptera he has 

 examined, and he considers that an analogous structure exists in 

 Insects of other Orders. He thinks it is concerned with the 

 perception of vibration, there being no sharp distinction between 

 auditory and tactile sensation. 1 



About one-half of the Diptera possess a peculiar structure 

 in the form of a head -vesicle called " ptilinum." In the fly 

 emerging from the pupa this appears as a bladder-like expansion 

 of the front of the head ; being susceptible of great distension, it 

 is useful in rupturing the hard shell in which the creature is then 

 enclosed. In the mature fly the ptilinum is completely intro- 

 verted, and can be found only by dissection ; a little space, the 

 " lunula," just under an arched suture, extending over the point 

 of insertion of the antenae remains, however, and offers evidence 

 of the existence of the ptilinum. This structure is also of 

 importance in classification, though, unfortunately, it is difficult 

 to verify. 2 



No point of Insect morphology has given rise to more differ- 

 ence of opinion than the mouth of Diptera ; and the subject is 

 still very far from being completely understood. The anatomy 

 and morphology of the mandibulate Insect-mouth are compara- 

 tively simple (though not without greater difficulties than are 

 usually appreciated) ; and it has been the desire of morphologists 

 to homologise the sucking mouth of Diptera with the biting 

 mouth ; hence the view that the appendages of three segments 

 are separate and distinct in the fly's mouth is taken for granted, 

 and it is further assumed that some of the secondary parts of the 

 appendages of the biting mouth can also be recognised in the 

 sucking mouth. The anatomy of the mouth-parts is, however, 



1 Zool. An~. xvii. 1894, p. 35, and Ann. Nat. Hist. (6) xiii. 1894, p. 372 ; Zcitschr. 

 wiss. Zool. Iviii. 1895, p. 475. 



2 Cf. Osten Sacken, Berlin, ent. Zeitschr. xxxviii. 1893 ; and Becher, JFien. ent. 

 Zcit. i. 1882, p. 49. For an account of the condition, with diagrammatic figures, 

 of the fly emerging from the pupa, cf. Sasatti, J. Coll. Japan, i. 1887. p. 34, 

 pi. vi. 



