4 



The AjDterygota are the most ji^eneralized of all 

 Insecta, and in their external form they resemble the 

 larvie of many of the Pterygota, or transient stages 

 in the larval development of others. A very marked 

 characteristic of the group is the existence of abdominal 

 appendages, either in the form of terminal processes at 

 the end of the body, or as processes in relation with the 

 sterna of the abdominal segments. The mouth-parts are 

 mandibulate, except in the few instances where they have 

 become imperfectly suctorial. 



It is true that wingless forms are by no means of 

 infrequent occurrence among other groups of Insecta, but 

 in them the apterous condition is a secondary acquisition, 

 and not an ancestral character, as is the case in the 

 Apterygota. Furthermore, maxillulee may also be present 

 in other insects, but in the few instances where such 

 are known to occur they are found only in a vestigeal 

 condition. 



The Collembola and Thysanura each possess certain 

 very marked characters, and are best regarded as two 

 sej)arate orders of insects. 



The Collembola are popularly known as " spring- 

 tails." In them the antennae have from four to six joints, 

 and the abdomen consists of six segments and is never 

 terminated by any appendages. The first abdominal 

 segment always bears a ventral tube, and the fourth 

 segment usually carries a springing organ. The mouth- 

 parts are always sunk within the head, the nervous 

 system consists of a brain and four ventral ganglia, and 

 there are no Malpighian tubes. 



The Thysaxuea are commonly known as " bristle- 

 tails." They have many-jointed antennae, and the 

 abdomen consists of ten segments and is provided with a 

 variable number of pairs of short, unmodified limbs. The 



