OF THE MILK-WEED BUTTERFLY. 9 



the oesophagus leads into the closed alimentary canal, it is evident that the former offers 

 the easiest route for a supply to fill the vacuum produced in the pharynx. 



The organ just described has hitherto escaped the notice of insect anatomists, and its 

 functions have therefore been conjecturally ascribed to other parts. The so-called " suck- 

 ing stomach" thus received its name from the earlier writers, and when its structure was 

 better known, and such a purpose negatived, the capillarity of the fine canal of the pro- 

 boscis, and even a peristaltic action of the latter have been suggested to explain the 

 power possessed by the butterfly to suck up its food.^ 



At the upper extremity of the pharynx opens the narrow oesophagus and at the lower 

 edge of the hypopharynx the common duct of the salivary glands discharges into the 

 canal of the proboscis at its base. These glands consist of two tubes arising in the base 

 of the abdomen, and passing, with many convolutions, on either side of the oesophagus, into 

 the head. The glandular portion (fig. 2, s. gl.) of these tubes is about 40 mm. long, and the 

 more slender anterior and non-glandular portion, or duct, {s.d.) about 12 nun. The two 

 ducts unite into one in the base of the head; the common duct, as just said, opening 

 below the hypopharynx. 



The oesojihagus (oe.) is a slender and delicate tube leadiiig from the pharynx above, and 

 after piercing the nerve commissure between the brain and the succeeding ganglion, 

 passes straight through the thorax into the abdomen, in the very base of which it opens, 

 above, into the food-reservoir, and below, and a little farther back, into the stomach. 

 The food-reservoir, {f.r.) or so-called sucking stomach, is a large, bladder-like sac, occu- 

 pying the upper part of the anterior half of the abdomen. Its walls are delicately mem- 

 branous, well supplied with longitudinal and transverse slender muscular fibres, particularly 

 the former. Clothing the upper surface of the interior, more thickly in the median line, 

 are long hair-like processes of the cuticle (pi. 2, fig. 12), whose points are directed for- 

 ward, that is towards the neck of the reservoir. These processes have broad corrugated 

 bases and end in long slender tips, appearing therefore, as if formed by being pulled out of 

 the membrane they cover, or as if each were a bunch of hairs clotted together at the tip. 

 Their use is not obvious. The food-reservoir is generally found to contain nothing but air ; 

 but Newport states that it is filled with food after feeding, and as it is not glandidar, it 

 probably serves simply as a reservoir for the temporary reception of food. The neck of 

 the reservoir is large, and by the contraction or extension of its muscular fibres, it is evi- 

 dent that food may be easily expelled from, or drawn into, its interior. 



Below and just behind the neck of the food-reservoir, the oesophagus opens into the 

 stomach (st.), a straight tube running along the ventral region of the abdomen from the 

 base of the second, into the fifth segment. Its walls are thick and composed of muscular 

 and glandular layers. The stomach is overlaid with the convolutions of the urinary, or 

 Malpighian, vessels, (see fig. 2, m.v.) six in number, three of which on either side unite 

 and open by a short common duct into the posterior end of the stomach. The urinary 

 tubes are about 90 mm. long. At the end of the stomach begins the small intesti?ie, 



1 For the previous literature of the mouth parts in Lepi- Kirby and Spence, etc. 



doptera, see Gerstfeldt, loc. cit. ; Milne Edwards, Physiol- Compare also the structure of the pharynx in Diptera and 



ogie; Newport, Phil.Trans., 1834, p. 397; Savigny,Memoires Hemiptera as described by Graber, Insecten i, 316, and 



sur les Aniraaux sans Vert., i ; Graber, Insecten, i, 154; Amtl. Ber. Vers, deutsch. Naturforschersamml. MUnchen, 



and the general works of Westwood, Burmeister, Siebold, 1877, p. 187. 



