Packard.] 290 



or probably the tergal portions in front of the mouth. Thus each re- 

 gion of the insectean body is characterized by the relative development 

 of the three elements of the arthromere. In the abdomen the upper 

 (tergite) and under surfaces (sternite) are most equally developed, while 

 the pleural line is reduced to a minimum. In the thorax the pleural 

 region is much more developed, either quite as much, or often more 

 than the upper or tergal portion, while the sternite is reduced to a 

 minimum. In the head the pleurites form the main bulk of the 

 reoion, the stcrnites are reduced to a minimum, and the tergites are 

 almost entirely aborted, or may perhaps be identified in the centre of 

 the "occiput," or what is probably the mandibular (or mandible-bear- 

 ing) ring, and in the "clypeus." 



In the abdomen the same abolescence of parts strikingly exem- 

 plifies what may be called the law of systolic growth, where certain 

 parts of the zoological elements of a body are in the coarse of devel- 

 opment either greatly enlarged over adjoining parts, or become wholly 

 obsolete, as stated by Audouin and St. Hilaire, who ascribed it to the 

 principle of "arrest of development," which is now used by physiolo- 

 gists in a more limited sense. While, as we have shown above, the 

 o-enital armor of insects is not homologous with the limbs, there are, 

 however, true jointed appendages attached to the ninth or tenth 

 abdominal rings, or both, which are often antcnnaj-form, and serve 

 as sensorio-genital organs in most neuroptera and orthoptera. The 

 abdominal limbs are confined as a rule to the two lower suborders 

 of insects, and are homologous with the "false legs" of the larva 

 of Lepidoptera, the abdominal legs of Myriapoda, and, we believe, 

 with the three pairs of abdominal appendages or spinnerets of the 

 Arachnids. As in the most anterior rings of the head, so in the 

 terminal abdominal rings, there only remain minute portions of the 

 arthromere, which are tergal pieces, the other two elements of the 

 rin*T being rarely present, or entirely aborted. The two opposite poles 

 of the body are therefore fashioned according to the same laws, and 

 are morphologically simply repetitions of each other. 



In conclusion, we consider that twenty rings (arthromeres), as a 

 rule, compose the bodies of insects, of which seven are contained in 

 the head, three in the thorax, and ten in the abdomen, and that as 

 thus grouped, forming three distinct regions, the insects differ from 

 all other articulates, standing as a class above the Crustacea and 

 Worms. The arachnids and myriapods, as Mr. Scudder* has shown, 

 agree with the Insects In possessing a distinct head separated from the 

 thorax or "pseudo cephalo-thorax," sothat the Myriapoda do not form 

 a class by themselves equivalent to the Crustacea, or Worms, or 



* These Proceedings, Vol. IX, p. 69. May, 1862. 



