48 



THE COCKROACH : 



the most dissimilar Insects, as to furnish a strong argument for 

 the descent of all higher Insects from forms not unlike Orthop- 

 tera in the structure of their mouth-parts. 



Though the jaws of the Cockroach are eminently primitive 

 with respect to those of most other Insects, they are themselves 

 derived from a far simpler arrangement, which is demonstrable 

 in all embryonic Insects. Fig. 21 shows an Aphis within the 



MX' 



MX' 



Fig. 21. Embryo of Aphis. Copied from Mecznikow, Zeits. f. wiss. Zool., 

 Bd. XVI., taf. xxx., fig. 30. References in text. X 220. 



egg. The rudiments of the antennae (At), mandibles (Mn\ and 

 maxilloo (Mis 1 , Mx 2 ) form simple blunt projections, similar to 

 each other and to the future thoracic legs (L 1 , L 2 , L 3 ). We 

 see, therefore, that all the appendages of an Insect are similar 

 in an early stage of growth ; and we may add that a Centipede, 

 a Scorpion, or a Spider would present very nearly the same 

 appearance in the same stage. A Crustacean in the egg would 

 not resemble an Insect or its own parent so closely.* Aquatic 

 life favours metamorphosis, and most Crustacea do not begin 

 life with their full quota of legs, but acquire them as they are 

 wanted. 



Paired appendages of perfectly simple form are therefore the 

 first stage through which all Insect-jaws must pass. Our second 

 stage is a little more complex, and not nearly so universal as the 

 first. A caterpillar (fig. 22) has its own special wants, and these 

 are met by the unequal development of its jaws. The mandibles 

 are already as complete as those of the Cockroach, which they 



* Freshwater Crustacea, however, are sometimes similar to their parents at the 

 time of hatching. 



