HYMENOPTERA. 



463 



curious phenomenon of nature. This parasite upon parasite is exem- 

 plified by the Colax dispar (fig. 1023). 



It is impossible to overrate the importance of the great family of 

 Ichneumonidae to the horticulturist. These winged insects deposit 

 their eggs, some in the caterpillar, others in the chrysalis. They then 

 turn into grubs, and devour the creatures in which they live, and thus 

 are good friends to the gardener by destroying his foes. 



In my work on the Potato plant I have mentioned that there 

 is a set of hymenopterous insects which 

 seize upon aphides and carry them off to 

 their habitation to feed their own young 

 ones. The Pemphredon unicolor (^g. 1024) 

 is an example of this tribe. 



The Saw-flies constitute another group 

 of hymenopterous insects injurious to the 

 garden. Their larvae resemble caterpillars, and destroy the leaves of 

 many plants. The larvae of Horn Tails bore the trunks of timber 

 trees. At my garden for several years, and especially in 1871, 

 we have been much troubled with the Rose Saw-fly {Hylotoma ros(£, 

 fig. 1025). It devours the leaf of the rose-tree, leaving one mem- 



Fig. 1024. — Pemphredon unicolor 

 (magnified 3 diam.)> 



Fig. 1025. — Rose-leaf and Larva 

 of Rose Saw-fly. 



Fig. 1026. — Tenthredo rosarum, 

 magnified. 



Fig. 1026a. — Pear-tree 

 Saw-fly. 



brane of the leaf only. It appeared in such quantities in some 

 parts of the garden that every leaf on many trees was utterly de- 

 stroyed at midsummer.* The Tenthredo rosarum (fig. 1026) is 

 represented by Boisduval to be very destructive to the rose ; and the 

 Pear-tree Saw-fly (fig. 10260:) sometimes inflicts much injury. 



The larva of the genus Sirex, 5". juvencus, has powerful mandibles, 



