54 THE REPORT OF THE No. 36 



THE IMMATURE STAGES OF THE TENTHREDINOIDEA.* 

 Alex. D. MacGillivbay^ University of Illinois, Urbana, III, 



An interest in the study of the adults of the Tenthredinoidea has emphasized 

 the necesity for some knowledge of the immature stages. The differentiation of 

 species from adult characters is at times a difficult one. It is frequently found 

 that species closely related and difficult to separate as adults have larvae that 

 are not only very different structurally, but also in their habits. It was hoped 

 from a study of the immature stages of the Tenthredinoidea that some information 

 might be obtained as to the va,lidity of the species based on obscure anatomical 

 details. This opportunity came through the offer of the Maine Agricultural 

 Experiment Station to collect, breed, and study the Tenthredinoidea of Maine 

 during the summer of 1913. 



In all phylogenetic studies the groups marking the beginning of things are 

 always not only of the greatest interest, but of the greatest importance, because 

 a study of their structures always shows something as to the origin of tlie struc- 

 tures found in the higher, more specialized groups of the same order. The 

 superfamily Tenthredinoidea is such an order, so that any morphological study 

 of the forms included within this group should be of particular , interest. It 

 contains the most generalized species of the order Hymenoptera, and has been 

 considered by all students of the order as the most primitive group. 



The adult insects of this superfamily have the proximal segments of the 

 abdomen similar in form and the abdomen broadly joined to the thorax as in the 

 beetles or locusts. Among all other groups of the Hymenoptera, the conditions 

 are very different. There is a deep constriction between the first and second 

 abdominal segments. The construction is so deep that many systematists have 

 considered this as the point of separation between the thorax and the abdomen. 

 The true first abdominal segment is completely fused with the meta thorax and 

 the spiracles borne by this segment, usually considered as the metathoracic spiracles, 

 are the spiracles of the first abdominal segment. This constriction between the 

 first and second abdominal segments of the adult arises during pupal life and 

 is used, to separate the Hymenoptera into two distinct groups. 



These groups are usually designated as suborders by systematists. To the 

 one including the Tenthredinoidea, the early students of the order gave the name 

 of Phytophaga, from the fact that tlieir larvse are all plant feeders. The later 

 workers, however, have adopted the name of Chalastogastra, based on the struc- 

 ture of the base of the abdomen. These terms are co-extensive with the super- 

 family name Tenthredinoidea as here used. 



The early systematists divided the group into two families, the Tenthredinidse 

 and Uroceridae, the saw-flies and the horn-tails. This grouping was followed for 

 many years, but has been quite generally discarded now. The English entomologist, 

 Peter Cameron, was the first to subdivide these families. He recognized four 

 families. Tenthredinidas, Cephid;r, Siricidffi, and Oryssida?, which were subdivided 

 into subfamilies and tribes. These additional families are a result of a splitting 

 of the family Urocerida^, now known as the Siricidse through the displacing of 

 the generic name Uroceriis by Sirex. 



*iContribution from the Entomological Laboratories of the University of Illinoia, 

 No. 38, and Papers from the Maine Agricultural Experiment Station: Enitomology No. 70. 



