348 THE BIOLOGY OF INSECTS 



of those appendages which can be seen on the embryonic 

 segments (Fig. 40, C) of many insects before hatching, suggest 

 that the ancestors of insects had paired limbs on all or 

 most of their segments, a condition persisting in the Chilo- 

 poda (centipedes), Diplopoda (millipedes), Symphyla, 

 Crustacea, and in the aquatic Arachnida (the living 

 horse-shoe ** crabs," Limulus, and the extinct Eurypterids). 

 In all classes of the Arthropoda a definite head consisting 

 of five or six limb-bearing segments is evident, except 

 among the Arachnida and those Crustacea in which this 

 region is fused with the segments behind it to form a 

 cephalothorax. The relationship of primitive Insects to 

 Crustacea is clearly shown by the similarity of the compound 

 eyes in the two classes, by the close likeness of the mandibles 

 of Thysanura and larval mayflies to those of various 

 Crustacea (Fig. 79, a, c, e), and by the presence in those and 

 other adult and larval insects of a pair of probably appen- 

 dicular lobes in front of the tongue, which represent either 

 the crustacean paragnaths or maxillulae (Fig. 79, /, g). 

 These evidences of kinship between insects and crus- 

 taceans have been strongly emphasized, though with 

 differences of opinion as to details, by H. J. Hansen (1893), 

 and C. Borner (1909), and G. C. Crampton (1921) 

 as well as by the writer of this book (Carpenter, 1903, 

 1905, 1924). To the points already mentioned may 

 be added the striking fact that in the Insecta, Arachnida, 

 and malacostracous Crustacea, as well as in the small but 

 highly interesting class of the Symphyla (whose members 

 combine characters of insects, centipedes, and millipedes) 

 there appears to be precisely the same number of segments 

 in the body. Such close numerical correspondence cannot 

 be a mere coincidence ; it must indicate relationship, and 

 it is interesting to note that T. H. Huxley nearly seventy 

 years ago (1859) commented on the great significance of 

 a cockroach, a lobster, and a scorpion possessing exactly 

 the same number of somites. We must await further 

 palaeontological evidence before offering any definite opinion 

 as to the group of Crustacea to which insects may be most 



