4 A7inaJs Entomological Society of America [Vol. XI, 



and 11th abdominal segments can no longer be determined 

 with certainty (text Fig. IB and Fig. 2). The terminal com- 

 pound ganglion {G7ig 9-11) has become shorter and thicker and 

 now lies almost entirely in the 9th abdominal segment. This 

 displacement is due to a lengthening of the trunk and not to an 

 actual shortening of the nerve chain. 



The formation of a ganglion — or pair of ganglia — in the 11th 

 abdominal segment also occurs in Lepisma (Heymons 1897), 

 Gryllotalpa, Peripla?ieta, Gryllus (Heymons 1895), Odonata and 

 Ephemerida (Heymons 1896), Leptifwtarsa (Doryphora) (Wheeler 

 1889), Donacia (Hirschler 1909), Hylotoma (Graber 1890) and 

 Chalicodoma (Carriere and Burger 1898). The fusion of the 

 terminal ganglia of the ventral cord to form a compound 

 ganglion is apparently general among insects. The number 

 of ganglia thus united varies, but appears in young larvae to 

 be usually three or four, more frequently the latter number. 

 Of the forms above listed, the Ephemerida, as well as embryos 

 of Hylotoma and Chalicodoma agree with the young honeybee 

 larva in having a terminal ganglion made up of three ganglia. 

 In all cases in which a ganglion rudiment is formed in the 11th 

 abdominal segment this rudiment is distinctly smaller than 

 the others and very usually forms only a vestigial, or at least 

 much reduced ganglion, as in the honeybee. Hirschler (1909) 

 makes the suggestion that in those species of Coleoptera, such 

 as Hydrophilus, in which the ganglion of the 11th abdominal 

 segment has not been observed, this ganglion rudiment has 

 suffered reduction to the point of disappearing altogether. 

 This assumption may of course be extended to other insects 

 than those of the order Coleoptera, such as Forficula, in which 

 Heymons (1895) could find no neuromere in the 11th abdominal 

 segment. 



At the time of hatching, the dorsal hypodermis shows no 

 evidence of an eleventh abdominal segment, only ten of these 

 being indicated by constrictions (text Fig. IB). Since the 

 formation of the dorsal hypodermis is completed only shortly 

 prior to hatching, it seems reasonable to conclude that only the 

 sternal part of the eleventh abdominal segment is present. 

 This, as is sufficiently evident, unites with the sternal part of 

 the 10th segment. A similar condition obtains according to 

 Heymons (1896) in the true Orthoptera, the Plecoptera and 

 some Odonata (imagines), the tergum of the 11th abdominal 

 segment being absent in these forms. 



