3i8 THE BIOLOGY OF INSECTS 



Sub-order i. SYMPHYTA 



The Sawflies have no constriction at the base of the 

 abdomen. Their larvae are caterpillars with seven or 

 eight pairs of unarmed abdominal pro-legs (Fig. 76). 



Sub-order ii. APOCRITA 



The Gallflies, Ichneumon-flies, Wasps (Fig. 60), 

 Bees, and Ants (Fig. 61) have a strong constriction 

 (*' waist ") behind the first abdominal segment. Their 

 larvae (Figs. 50, 62) are legless grubs. 



In the above summaries of the orders of insects reference 

 has been made to families that are comprised in certain of 

 the orders. A family in a zoological classificatory system 

 is thus a group subsidiar}^ to the order. It is convenient 

 to divide some of the orders (Hemiptera and Hymenoptera, 

 for example) into sub-orders, and many students group 

 together a number of related families into an assemblage 

 called a tribe or a superfamily, intermediate between the 

 order or sub- order and the family. An enumeration of all 

 the families of insects is impossible in this book, but it may 

 be well to follow up our summary of the insect orders and 

 their distinctive characters by giving a few illustrations of 

 the kind of characters on which families are based. These 

 illustrations may be conveniently taken from the Hymeno- 

 ptera, the latest order in the summary. 



There are two well-marked sub-orders of Hymenoptera 

 distinguished by structural features both in the adult and 

 larval stages. Of these the Symphyta are a relatively small 

 group including only four or five families, w^hile the 

 Apocrita are a very large and dominant group including 

 twenty families or more. 



Among the Symphyta the two most important families 

 may suffice. The Tenthredinidae (Sawflies) comprise 

 several thousand species of insects which, in addition to 

 the sub-ordinal character of the broad-based abdomen, 



