1909.] NATURAL SCIENCES OF PHILADELPHIA. 7 



as Julus not only the neuromeres, but also the cardiac ostia, arteries, 

 tracheae and legs plainly show the double nature of the somites. Other 

 indications of segmental fusion are two pairs of tracheal invaginations 

 in each segment of Acilius, the bifurcated appendages of many Crus- 

 tacea, and the bifid maxillae of insects, in which latter group monsters 

 with double pairs of legs are of frequent occurrence. 



All of the subsequent theories, though differing greatly in their 

 point of view, lay great stress upon the fusion of segments traceable 

 in the Myriopoda as an indication of what has taken place in the 

 Hexapoda. 



Banks, '93, regards the suture between the episternum and epimeron 

 as the boundary between two leg-bearing subsegments. Of these, the 

 anterior or episternal subsegment, has retained its appendage fully 

 developed, while the leg of the posterior or epimeral subsegment 

 occurs only in a vestigial condition (the so-called styli found on the 

 meso-, and meta-thorax of Machilis and other insects) or is completely 

 fused with the episternal leg. 



Walton's, '00-01, theory differs from that of Banks only in the fact 

 that he regards the epimeral leg as represented by the so-called meron 

 or posterior portion of the coxa, and in that he believes that each sub- 

 segment originally bore a wing. According to this author, only the 

 epimeral wing is fully developed, while that of the episternal subseg- 

 ment exists only as a wing "fundament," and is represented by the 

 squamulae, tegulae, etc., designated under the common term pterygoda. 



Kolbe, '93, whose book appeared contemporaneously with Banks', 

 '93, first publication, differs from Banks and Walton, in regarding the 

 epimeron and episternum as parts of the same segment, and in addition 

 he finds traces of other " complementary" segments in such forms as 

 Locusta, (Edipoda, etc. These complementary segments are especially 

 well developed in the larvae of Lampyris, and here show a great simi- 

 larity to the condition found in Scolopendrella — which Kolbe considers 

 as an intermediate form between the rest of the myriopods and insects. 



Verhoeff, '02-04, accepted Kolbe's theory, which he enlarged and 

 worked out more in detail. Believing that traces of three "Vorder- 

 segmente" or complementary segments (one in front of the pro-, meso-, 

 and meta-thorax respectively) are to be found in such insects as 

 Japyx, Embia, etc., he proposes that the typical hexapod thorax is 

 composed of six primitive segments. To the " Vordersegmente " he 

 gives the names micro-, steno- and crypto-thorax. Of these, the 

 microthorax (the complementary segment in front of the prothorax) 

 is the best developed, and occurs in a large number of insects. In 



