LECTURES ON EMBRYOLOGY. 



59 



[PLATE XIII ALIMENTARY CANAL OF BUTTER- 

 FLIES ] 



general arrangement they are all tubular, thread- 

 like, and very long. 



The next glandular apparatus here (Plate XIII. 

 fig. A.,) is the gland seen on each side, behind the 

 salivary tubes, the silk glands, which are much 

 larger in the Caterpillar than in the perfect in- 

 sect. These silk glands still exist in the perfect 

 insect, but they are much larger in the Caterpillar 

 than in the Pupa, and again larger in the Pupa 

 than in the perfect insect. You are aware that the 

 Caterpillar draws its silk from its mouth, winds it 

 regularly around its body, to protect it during its 

 second stage of metamorphosis. The third gland- 

 ular apparatus, a kind of liver, consisting of three 

 pairs of hepatic tubes, emptying in the posterior 

 part of the wide tube of the Caterpillar, but about 

 its middle in the perfect insect. This condition 

 of the glands, which we find among all 

 the Insects, is far from the structure 

 of those massive glandular organs which 

 occur in other animals. The lower portion of the 

 alimentary canal is scarcely at all contracted, in 

 the Caterpillar, as you will observe in this figure 

 (Plate XIII., fig. A ) Before entering the papa 

 state (Piate XIII., fig. B ,) at a period when the 

 insect is more perfect, the cesophagus has become 

 narrower and longer; and the colon has also 

 become more elongated and narrower, and in the 

 pupa state you see how the digestive tubes appear. 



(Plate XIII., figrue C.) The animal has now 

 ceased to take food, and the salivary glands dis- 

 appear entirely. (Plate XIII., fig. D ) Next, the 

 colon grows more slender, to be transformed into 

 a narrow cylindric tube. When the Pupa is ready 

 to be transformed (Plate XIII., fig. E ,) into a But- 

 terfly, there is a new pouch formed between the 

 cesophagus and stomach, a pouch which secretes 

 the honey. It is a sack, to produce the sweet fluids 

 which so many insects are capable of secreting, or 

 at least of preparing. This pouch (Plate XIII., 

 fig. E ,) has grown to a somewhat large size, and 

 the posterior part of the alimentary canal has been 

 elongated very considerably, in proportion as the 

 middle part or the stomach proper has been re- 

 duced. And finally, in the Butterfly, it is fully 

 developed, but we see no longer any salivary 

 glands. (Plate XIII., fig. F.) The posterior part 

 of the alimentary canal is now long and slender, 

 and the hepatic duct of the liver nearly as large 

 and as complicated as in the beginning. 



Here again, we see that in proportion as the ali- 

 mentary tube is a uniform tube or in proportion 

 as there are cavities of different diameters devel- 

 oping along its longitudinal diameter we have 

 another scale to determine the relative rank of an- 

 imals in which this organization is observed. 



[PLATE XIV. LONGITUDINAL SECTION OF 

 SPHINX LIGUSTRI.] 



This is, perhaps, better seen in another diagram of 

 a Moth, where we see the cesophagus passing 

 through the anterior nervous ring, and extending 

 in the perfect insect PI. XIV. (Fig. A) through the 

 chest, where the wings are cut off and the legs 

 also. The large thorax answers to that part of 

 the Caterpillar (Figure B,) where the horny 

 legs are seen, and the ganglionic portion of the 

 nervous system is seen all along below the alimen- 

 tary canal. And in the Caterpillar you see how 

 intimately and uniformly the nervous swellings 

 follow each other, (Plate XIV, fig. B) and how the 

 alimentary canal is a uniform tube, whilst in the 

 perfect insect, alimentary canal and nervous sys- 

 tem have undergone remarkable concentrations 

 (Fig. A). 



Another apparatus is very simple among Insects. 

 It is one of those functions which is not so high- 

 ly developed as in other Articulata, but which, 



