CHAPTER XI 

 THE APTERYGOTA 



These are all relatively small insects, some being nearly microscopic 

 in size, while the largest are seldom more than an inch in length. They 

 are all land animals though a few live near the ocean and are occasion- 

 ally found in tide pools. They are widely distributed over the earth, 

 some living in arctic conditions while others occur in the tropics, but 

 nearly all at least, require a somewhat humid atmosphere. 



In this group the mouth parts seem to be typically of the chewing 

 type. In many cases they are as much exposed as in most insects, but 

 in some they appear to have been drawn into the head so that when not 

 in use they are almost entirely concealed. Under such conditions they 

 are often so slender as to be no longer of value for chewing, and are 

 possibly used for rasping and sucking food. 



Some Apterygota have traces of abdominal legs ("vestigial legs"). 

 Spine-like appendages, attached to the hinder margins of some of the 

 abdominal segments beneath and called styli, may also occur. A 

 ventral tube present in some Apterygota on the underside of the first 

 abdominal segment may be simply a small projection partially divided 

 into two, or it may be highly developed into two slender but delicate 

 tubes which can be extended to a considerable distance. Its use is not 

 known. 



Bringing together these facts, the Apterygota may be characterized 

 as: 



Wingless insects having the mouth parts either exposed and of the chewing 

 type, or drawn into a cavity within the head where they are sometimes so 

 slender as to be of no value for chewing but could possibly be used for sucking. 

 In those with exposed mouth parts, more then one pair of styli is present on 

 the back margins of the hinder abdominal segments: in those with mouth 

 parts drawn into the head, styli, a ventral tube or traces of abdominal legs 

 are present. 



Very few of the Apterygota are of any importance from an economic 

 standpoint, but they are of much interest, being the simplest insects 

 known and throwing some light upon the subject of the ancestry of the 

 insect group. 



Two subdivisions, the orders Thysanura and Collembola, are generally 

 recognized in the Apterygota. The Thysanura have styli present, 

 while in the Collembola they are absent. Cerci, which are segmented, 



62 



