8 PROC. ENT. SOC. WASH., VOL. 23, NO. 1, JAN., 1921 



of his interpretations have been taken seriously. But if we 

 could adopt Mecznikov's statements as true, the problem of 

 interpreting the Hemipteran head and mouth parts would be 

 greatly simplified. 



Unfortunately for our peace of mind, however, no subsequent 

 embryologist has agreed with Mecznikov, while all have agreed 

 with one another that the setae are developments of the true 

 mandibular and maxillary embryonic appendages. Witlaczil 

 (1882) stated that it is not difficult to show that the mandibles 

 and first maxillae are not lost in development, as described by 

 Mecznikov, but, sinking into the head, form the retort organs 

 from which the setae are chitinous outgrowths. Heymons (1899) 

 showed that the mandibular appendages of the embryo are 

 developed entirely into the first setae, but that each maxillary 

 appendage divides at an early stage into a basal part and a 

 distal part, the first fusing with the lateral wall of the head, the 

 other becoming the second seta. Muir and Kershaw (1911, 

 1912) have verified Heymons' statements in both the Homop- 

 tera and the Heteroptera. All writers, except Smith, agree that 

 the labium of the adult is formed from the fused labial appen- 

 dages of the embryo. 



Therefore, we must either admit that the Hemipteran mouth 

 setae are the true mandibles and maxillae, or we must produce 

 evidence to show that the more recent students of Hemipteran 

 embryology are mistaken in their facts. Up to the present 

 Mecznikov is discredited, and all direct anatomical comparisons 

 of the Hemipteran with the Orthopteran head fail to give satis- 

 faction. To adjust our ideas we must begin by assuming that 

 the bases of the mandibles in the Hemiptera have moved 

 posteriorly across the bridge of the hypopharynx (fig. 7, a, a] 

 and have sunken into pouches, along with the maxillae, behind 

 the latter. Heymons, in one of his figures, appears to indicate 

 that this migration of the mandibles has taken place, though he 

 does not specifically describe it. 



It would seem then, that the embryological evidence should 

 end all discussion concerning the homologies of the setse, but 

 when we examine the details of their external connections with 

 the head, their internal apodemes, and the muscles that move 

 them, and then attempt to reconcile these features with those 

 of other insects a whole new set ot troubles confronts us. Muir 

 and Kershaw (191 la), discovering that the first seta on each 

 side has a basal arm (figs. 13, 14,/) reaching clear up to the dorsal 

 end of sclerite A on the floor of the deep sulcus between sclerite 

 A and B, established this point (g) as the true mandibular 

 articulation (anterior) with the head. Furthermore, they sub- 

 stantiate this interpretation with the fact that the invagination 

 for the anterior arm of the tentorium (ten) is located just above 

 the end of the setal arm. The same view has been adopted by 



