144 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



Imms (1936), who shows that the most plausible concept of the 

 ancestry of insects is that of symphylid derivation. The important 

 difference between modern Hexapoda and Symphyla is in the position 

 of the genital openings, the symphylids being progoneate, the hexa- 

 pods opisthogoneate. It is necessary to assume, therefore, that the 

 Protohexapoda were evolved from an opisthogoneate branch of the 

 Protosymphyla. 



The Protohexapoda became differentiated as a hexapod group 

 through the concentration of the locomotor function in the first three 

 postcephalic segments, with the consequent division of the body into 

 a motor thorax and a visceral abdomen. The abdominal appendages 

 were reduced, modified for purposes other than locomotion, or sup- 

 pressed, but in most cases the abdominal coxal remnants united with 

 the sternal plates of the segments and preserved the styli and eversible 

 vesicles inherited from the Protosymphyla, though on the thorax these 

 structures were lost. The number of body segments was limited to 

 14 somites and a simple terminal lobe (telson) containing the anus. 

 The persistent appendicular organs of the last somite were styluslike 

 cerci, as in Symphyla. It is probable that the true telopodites of all 

 the abdominal segments were absent. The mandibles became solid 

 jaws by a complete fusion of the lacinial lobes with the coxopodites, 

 and thus came to resemble the mandibles of Crustacea, but the 

 maxillae and labium retained the generalized protosymphylan struc- 

 ture. The hypopharynx consisted of a median lobe and two lateral 

 lobes, as in Symphyla, and had a pair of basal apodemes giving attach- 

 ment to muscles of the gnathal appendages. The eyes were compound. 

 The protohexapods were opisthogoneate insofar as the paired genital 

 apertures were located on the posterior part of the abdomen, but the 

 exact position of the ducts and their outlets was still subject to 

 mutation, as shown in the variable position of the genital outlets in 

 modern forms. 



The discrepancy in the position of the genital openings as between 

 Symphyla and Hexapoda raises the chief difficulty in relating the 

 hexapods directly to the symphylids. The opisthogoneatism of the 

 Hexapoda, however, is more truly a heterogoneate condition, which 

 in a broad sense applies to the entire group of labiate mandibulates, 

 for the primary genital ducts open on the third postcephalic somite 

 in Diplopoda and Pauropoda, on the eighth in Collembola, on the 

 tenth in female Pterygota, on the thirteenth (primitively) in male 

 Pterygota, and on the fourteenth in Protura. Since the primary 

 gonopores of the hexapods are always fixed with specific segments, 

 as in Symphyla, Pauropoda, and Diplopoda, the opisthogoneate con- 



