586 THE SENSES AND SENSORY ORGANS. 
third joint. Its outer surface bears several stiff setae, one of which is longer 
than the rest. It also has sensory sete onits 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 Muscide. 
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 sete. 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. 
DESCRIPTION OF PLATE XLI. 
The Antenna and Palpus of the Blow-fly. 
Fic. 1.—A longitudinal antero-posterior section of the antenna of a Blow-fly. 
a', a’, and a’, the first, second, and third joints ; g, ganglionic enlargement of 
the antennal nerve in the second joint ; #, antennal nerve ; s, s, sacculi on the 
posterior aspect of the third joint. 
Fic. 2.—Sections of the integument and subjacent ganglion cells of the same, seen 
with a 4’, oil immersion lens. A, a portion of the surface of the antenna. 
g, ganglion cells; 4, hypoderm cells; #5, protective sete; ss, sensory seta. 
B, one of the sacculi ; s, orifice of thesac. C, Flame-like process (/), or cone, left 
after the detachment of a seta; g, ganglion cell, partly situated in the subjacent 
pore canal. 
Fic. 3.—A portion of one of the maxillary palpi. 7, nerve; ¢, trachea. 
Fic. 4.—A portion of the integument of the palpus seen with an oil immersion jy 
objective. g, ganglion cells ; 4, hypoderm ; 7, nerve; ss, sensory sete. 
