26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



bear a pair of clavate organs (pi. 8, fig. 25) similar in shape and 

 position to those on the basal segment of the legs. Kenyon refers to 

 these structures as " papilliform processes," but the present writer 

 regards them as exceedingly short, cylindrical segments, so short in 

 fact that they are like discs. 



EVIDENCE OF A CERVICAL SEGMENT AMONG INSECTS 



But we do not have to turn to Symphyla or to Pauropoda to find 

 structures that indicate the existence of a cervical segment. In Japyx, 

 as is indicated by Snodgrass' drawing (Snodgrass, 1927, p. 16), there 

 is a repetition of the Y-shaped thoracic sternal apodeme on the ventral 

 aspect of the neck. Enderlein (1907) has figured this structure for 

 Japyx japonicus, but he regarded it as l^eing comparable to the seg- 

 mental folds of the meso-, and metathorax and called the same 

 a prothoracic apotom. The writer has made a special examination of 

 it and here gives a detailed drawing of the same (pi. 9, fig. 26). The 

 Japyx species studied is the large eastern Japyx bidentatus. This 

 Y-shaped sternal apodeme is indeed very similar to the one found 

 on each of the three thoracic sterna. From the fork of the Y, a 

 median process passes backward, as in the case with the others, to 

 the base of the following sclerite. The branches of the Y, however, 

 do not end in a headlike condyle as in the case of the thoracic apodemes 

 but become fused with the lateral angles of the included sternite, and 

 the sternite and the apodemes together form a more or less hingelike 

 fulcrum for the head. 



In Japyx isabellae (pi. 12, fig. 38) no cervical Y-shaped apodemes 

 are noted, but the cervical region is well developed. In; this species 

 the same sternite that is included between the forks of the Y in biden- 

 tatus is enlarged and strengthened. This sternite bears an articulating 

 surface with a condyle from the head at the median line. Thus a 

 similar function is performed in the absence of any apodemes. 



THE SEGMENTS INCLUDED IN THE INSECT THORAX 



It might appear, therefore, that the insect thorax is composed of 

 the second, third, and fourth body segments behind the head, and, 

 if so, further evidence would be found in some of the primitive 

 insects. The most primitive group of arthropods to show the thorax 

 developed is the Protura. In the Protura we have true hexapods, 

 although in this group a number of myriapod charatters are retained. 



Unfortunately in proturans the first pair of legs have been de- 

 veloped as feelers, and the prothorax accordingly modified and re- 



