Specific Characters 



143 



flat with convex upper surface, and shining reddish- 

 brown. The feelers are composed of eleven segments. 

 In one of the species — TrihoUum confusum — these seg- 

 ments gradually increase in breadth from base to tip 

 of the feeler, and the head is widened in front of the 

 eyes (fig. 85 a, e). In the other — T. ferrugineum 

 — the three terminal segments of the feeler are 

 markedly wider than the rest, forming a club, while 

 the head is not widened in front of the eyes (fig. 

 85 f). Moreover, the wing-cases are less deeply 



Fig. 83. — Flour-Beetles, a. TriboUum con/usuin ; l>. larva; c. pupa; d. pro- 

 cess of abdominal segment ; e. head of beetle \ /. head of T. ferrugineutn. 

 Magnified. From Chittenden, Bull. 4 (n.s.) Div. Ent. U.S. Dept. Agr. 



punctured in T. confusum than in T. ferrugineum. 



Now it must be admitted that such differences as 

 these seem to have no relation to the insects' success 

 in the struggle for existence. Moreover, as the habits 

 and surroundings of these two species are precisely 

 alike, it is hard to say why they should differ at all 

 if natural selection be the only cause of specific differ- 

 ences. Yet we know far too little about the relations 

 that may exist between the smaller details of structure 



