NO. 10 MORPHOLOGY OF GNATHOTRICHUS — SCHEDL 3I 



ments, the dorsal plate, or tergum, the ventral plate, or sternum, and 

 the lateral area, or pleuron. The thoracic segments and their elements 

 vary considerably in shape and size. The protergum, or pronotum, is 

 nearly twice as long as the tergum of the metathorax. The mesoter- 

 gum, still shorter than the latter, is represented by a short triangular 

 area only. The prosternum is about half as long as the metasternum 

 and about one-third the length of the pronotum, giving the pleural 

 area of the prothorax the shape of a trai^ezium. The mesosternum is 

 present as a plate nearly equal in size to the mesotergum ; the meso- 

 pleura are also developed in proportion, giving the mesothorax the 

 shape of a short tube. The metathorax, more complicated in struc- 

 ture, represents the segment in which all three primitive elements are 

 well developed and defined. 



THE PROTHORAX 



In the prothorax the tergal, pleural and sternal areas (fig. 14) arc 

 fused, forming a continuous chitinous tube. However, corresponding 

 elements to other thoracic segments are visible due to the different 

 kinds of sculpture. Taxonomically the prothorax bears generic char- 

 acters but none of specific importance. The two oj)enings of the tube- 

 like prothorax, the anterior and posterior foramina, are bordered by a 

 fringe of closely placed hairs which arise from the inner margin. 



Pronotum. — The pronotum (figs. i. 2. 3 and 14) is one solid plate 

 by which the head is concealed. The dimensions are as follows : 



Gn. iiiafcriariiis Fitch., length, 1.23 mm., width, 0.98 mm. 

 Gn. retiisits Lee, length, 1.44 mm., width, 1.15 mm. 



Gn. sulcatiis Lee, length, 1.35 mm., width, 1.07 mm. 



The measurements are the average of 10 specimens from each 

 species. 



Seen from above, the sides of the pronotum are subparallel on the 

 posterior margin, while the anterior margin is broadly rounded. The 

 anterior margin bears 10 to 16 low serrations which are only slightly 

 longer toward the median line and sometimes fused at their base. 

 On the anterior area, that is, the area in front of the summit which 

 consists of a short slightly curved ridge and is placed a short distance 

 before the middle of the pronotum, occur many comparatively small 

 and low serrations arranged in concentric rows and decreasing in size 

 towards the summit. Behind the summit the surface of the pronotum 

 is covered with punctures. There arc two kinds of punctures; the 

 one comparatively coarse in contrast to the other but fine compared 

 to those in other nearly related genera; the other, minute and only 



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