INSECTS. 



91 



adapted for varying purposes. Thus in the spiders, the spinnerets, and in the true 

 insects, the ovipositor or sting has been shown to be in reality composed of modified 

 appendages homologous Avith those on the other segments of the body. These greatly- 

 modified abdominal legs do not occur on all the segments of the adult, but in the larval 

 stages most insects have these ajjpendages developed to a greater or less extent, and 

 frequently of use in locomotion. Even here frequently some of the segments are with- 

 out well-marked limbs. The discussion of these changes produced by metamorphosis 

 will be resumed when treating of the sub-classes and orders. 



The nervous system of insects is constructed upon the same plan which, in the 

 preceding pages, we have found to be common to all Arthropoda. There is an enlarge- 

 ment in front of the oesophagus, the so-called brain, which is connected with a longer 

 or shorter chain of ventral ganglia, or nervous centres, behind that tube. In Peripatus 

 this ventral chain consists of two widely-separated nervous cords connected by numer- 

 ous fine filaments, and as the main cords are without well-marked enlargements or 

 ganglia the ventral chain closely resembles a ladder. In the myriapods the nervous 

 system most closely approaches what is considered as the typical condition, there being 



Fig. 128. — Anatomy of a butterfly, a. Anus. ao. Aorta, 

 duct. /. Food reservoir, k. Heart, m, Malpigliian vessel; 

 s. Stomach, sg. Salivary gland. s^\ Suckiug stomach. 



b. Brain, c. Colon, cp. Copulatory pouch, d. Ovi- 

 ■'. n. Nervous cord. a. Ovary, oe CEsophagus. 



a compound ganglion in each of the segments of the body. In the six-footed insects 

 the ventral chain is usually abbreviated to a certain extent Ijy a fusion of some of the 

 ganglia ; and in the Araehnida this reduction is frequently carried out to tlie greatest 

 degree, there being in some forms but a single compound ganglion behind the oesoph- 

 agus, while in others the brain almost entirely disappears. 



Besides the nervous system thus briefly described, there are two other portions well 

 develojjed. One of these supplies the alimentary tract, and is called the sympathetic 

 from its analogy with a system in the human body with a similar distribution and the 

 same name. The other is distributed to the respiratory organs, giving off branches to 

 the spiracles and trachea. 



The digestive canal of insects, like that of all animals above the Coelenterata, is 

 divided into three regions; the middle portion being formed from the primitive 

 embryonic stomach, while the anterior and posterior portions are ])roduced by the 

 subsequent pushing in or invagination of portions of the outer embryonic layer until 

 they meet and join the middle portion (see fig. 11). A distinction is however to be 

 noticed here between the insects and the crustaceans, as in the latter the primitive 

 stomach is produced by a true invagination, a regular gastrula being produced, while 



