AMERICAN ONISCOID DIPLOPODA OF THE ORDER 

 MEROCHETA. 



By O. F. Cook, 



Custodian of Myriapoda. 



In July, 1890, I collected near Auburn, Alabama, a small oniscoid 

 diplopod the affinities of which have been diflicnlt of deteiiniuation. 

 It is quite similar to a sjjecies described from Arkansas by Bollmau as 

 Sphcvriodesmus pudicus. The true geuus Sphcvriodesmus is, however, 

 a vety different form, and among described genera the type under 

 discussion approximates rather to Cyclodesmus, but otters diflerences 

 apparently important and at least unique. 



Outside the Merocheta are to be found adaptations for securing 

 safety by coiling up, conspicuously in the Onisconiorpha and in the 

 family Striariidic of the C(j'locheta, where the first segment is produced 

 in front into a hirge hood for the protection of the head. In the Onis- 

 comori)ha the specialization for the habit referred to has reached its 

 liigliest development, and the primitive condition of the ventral pro- 

 tected i)art8 is evidence that this adaptation dates well back in the 

 history of that order. 



In the liO segmented Merocheta the modifications necessary to render 

 the habit of coiling up an effective means of defense have been exe- 

 cuted in spite of greater initial difficulty, since the segments had, by 

 the coalescence of all primitive sutures, become solid and inflexible 

 chitinous rings. Moreover, this protective scheme seems to have been 

 carried out, not once merely, but several times independently, for it 

 appears from a comparative study of the five genera enumerated by 

 Latzel us composing the subfamily Si)ha'riodesmia that these have noth- 

 ing in common except this power of coiling closely. As conspicuous 

 proof of the truth of this view, there need be noted only the fact that 

 while some of the anterior segments are in each case enlarged to com- 

 I)lete the armor of the closed animal, there have been at least three 

 different inventions, if the expression may be permitted, to serve the 

 same purpose. In Oniscodesmus and Cyrtodesmns the second segment 

 is enlarged, in Cyclodesnius the third segment, and in Sphcvriodenvius 

 the fourth and fifth, as may be better understood from reference to the 

 plates. But even Oniscodesmus and Cyrtodesmn,s are widely distinct in 

 all their characters, and the enlarge<l second segments do not resemble 

 each other, showing that even where the same segment has been modi- 

 tied the histories of the changes may have been entirely independent. 



Proceedings U. S. National Museum, Vol. XXI-No. 1154. 



451 



