680 
.T. DAVIDSON. 
forced along the pump shaft into a fine canal down the fused 
maxillEe. 
The (Esophagus. — The oesophagus (u-s.) leads as a narrow, 
straight tube from the posterior end of the pharynx, and 
passing through the circum-oesophageal ring, over the 
transverse bar, extends in the median line above the thoracic 
ganglia. 
In the raesothorax it enters into the stomach or dilated 
region of the mid-gut.^ When examined as a fresh prepa- 
ration in normal salt solution, it appears as a whitish-grey, 
semi-transparent tube, with comparatively thick walls, which 
surround a very small lumen. 
The walls of the oesophagus consist of an epithelial layer 
composed of granular protoplasm, in which are scattered 
small, elongate nuclei, but cell boundaries are not Avell 
defined. A thin, outer coat of connective tissue sui-rounds 
the epithelium. 
The mid-gut. — The mid-gut extends somewhat obliquely 
in the median line, its anterior portion being considerably 
enlarged to form the stomach, which is the widest part of the 
alimentary canal. As will be seen in longitudinal sections, the 
oesophagus is invaginated for some distance into the stomach, 
its walls being reflected back to become continuous with the 
walls of that structure, thus forming an effective oesophageal 
‘ This expanded portion of the digestive tract in aphids is sometimes 
referred to as the “ crop.” The crop in insects is associated with the 
stomodceum, being derived from ectoderm, and lined with a chitinous 
intima. The cells composing the wall of the expanded portion of the 
mid-gut in S. lanigera agree in character with those in other parts of 
the wall of the mid-gut. Further, the place where the oesophagus termi- 
nates and the large cells of the expanded portion of the gut commence 
is clearly defined. There is no chitinous intima in this region. Of 
coi;rse, the absence of a chitinous intima is not in itself proof of its 
mesodermal origin, as a chitinous lining is found in the mid-gut of many 
insects, as shown by Schneider, but in these cases its origin is doubtful. 
This structure must be considered as part of the mid-gut, and the term 
“crop” is erroneous. I shall refer to this portion of the gut embracing 
the CESophageal valve as the stomach. It is the “ Magen ” of Dreyfus 
(1894) and others. 
