676 
J. DAVIDSON. 
I 
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As tliey enter the buccal cavity' they fuse to form a single 
chitinous seta, which extends between the mandibles along 
the proboscis groove and encloses two minute canals. 
Inserted on the internal face, at the proximal end of each 
of the mandibular setae, is a strong mandibular retractor muscle 
(mh m.), both of which extend in a postero-dorsal dii-ection 
to become attached respectively to the right and left antero- 
dorsal rods of the endoskeleton, near the ends of the trans- 
verse bar. A similar maxillary retractor muscle is attached to 
each of the maxillary setae (m'. mx.), and extend to the post- 
ventral rods, being attached to them near the ends of the 
transverse bar. 
The Retort-shaped Organs. — Extending from the 
proximal end of each of the setae is an elongate, compact 
mass of small cells possessing well-stained nuclei. They ai-e 
shown in fig. 22 [ind. o.), {mx. o.). These structures are 
related to the peculiar, retort-shaped organs {re. o.) found lying- 
in the thorax and posterior part of the head in well-developed 
embryos of S. lanigera and other aphids. In advanced 
embryonic stages they are large and conspicuous, being of a 
characteristic retort shape. From the neck of each retort a 
long, fine, chitinous tube, the so-called “ seta,” is produced, 
which may frequently be seen coiled round these organs in 
the anterior part of the embryo. 
These retort-shaped structures are bounded externally by 
a chitinous membrane and an epithelial layer of cells, with 
elongate, flattened nuclei ; and the interior of the retoi-t is 
])acked with a mass of small cells possessing deeply staining 
nuclei. As development proceeds from the larval stage to the 
adult they appear to degenerate, so that in the adult apterous 
viviparous female, after completion of the moultings, they are 
sometimes difficult to make out. Krassilstschik (1893, p. 9) 
says that in fully developed adults of Phylloxera vastatrix 
the retorts completely disappear. I have observed them in 
all the longitudinal sections of the apterous viviparous females 
of S. lanigera examined, although they usually appeared to 
be in a degenerate condition. 
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