$86 THE SENSES AND SENSORY ORGANS. 



third joint. Its outer surface bears several stiff seta:, one of which is longer 

 than the rest. It also has sensory set;o on its anterior and external surface 

 in relation with a special ganglion, which has usually been regarded as a 

 chordotonal organ. It is certainly very similar to Miiller's organ (see 

 p. 592), and is possibly an exceedingly rudimentary auditory structure. 



The cavity of the second joint communicates freely with the first joint, but 

 only by a comparatively narrow foramen (Fig. 72, D) with that of the 

 third joint. It contains a ganglionic enlargement of the antennal nerve. 



The third joint is ovoid and obscurely triquetrous ; the in- 

 tegument is thinner than that of the other joints; about 

 eighty large sacculi open on its surface by large irregular 

 pores, chiefly on its posterior, or facial, and inner aspects. In 

 Syrphus the sacculi are replaced by one or more long con- 

 voluted canals, and this also appears to be the case in many 

 of the Muscidae. 



The fourth and fifth joints are mere rings at the base of the 

 seta, which is inserted on the outer side of the third joint. 

 The seta is plumose ; the lateral appendages of the feather 

 are scales with a greatly elongated apex, and do not appear to 

 be hollow setae. I have been unable to trace a special nerve 

 to this structure, and its cavity is occupied by a blood sinus 

 and a tracheal vessel. As it is lined by a distinct hypodermal 

 layer, it is probably supplied by fine cutaneous nerves : but it 

 is not apparently the seat of any special nerve-terminals. It 

 is probably a mere persistent rudiment of the terminal joints 

 of the antenna. 



DKSCRHTION o\- PLAIT. XLI. 



The Antenna and Palpus of the Blow-fly. 



FIG. i. A longitudinal antero-posterior section of the antenna of a Blow-fly. 



a 1 , a-, and <z 3 , the first, second, and third joints ; , ganglionic enlargement of 



the antennal nerve in the second joint ; ;/, antennal nerve ; s, s, sacculi on the 



posterior aspect of the third joint. 

 FlG. -'. Sections of the integument and subjacent ganglion cells of the same, seen 



with a ,'., oil immersion lens. A, a portion of the surface of the antenna. 



.f, ganglion cells ; //, hypoderm cells ; / s, protective seUv ; s s, sensory setre. 



I'., one of the sacculi ; s, orifice of the sac. C, Flame-like process (/'), or cone, left 



after the detachment of a seta ; ,v, ganglion cell, partly situated in the subjacent 



I .ore canal, 

 l-'n;. .}. A portion of one of the maxillary palpi. ;/, nerve ;//, trachea. 



i. 4. A portion of the integument of the palpus seen with an oil immersion L '.T 



objective. , ganglion cells ; //, hypoderm ; >/, nerve ; s s, sensory sehv. 



