Wasps, Bees, and Ants 467 



and place an egg at the bottom of each. One of the best-known horntails 

 is the pigeon-tremex, Tremex columba (Fig. 654), ij inches long, with reddish 

 head and thorax and black abdomen with yellow bands and spots along 

 the sides. The females bore holes 

 inch deep into elms, oaks, sycamore- 

 or maple-trees, the ovipositor, in boring, 

 being held bent at right angles with 

 the abdomen. The larvae hatching 

 from the eggs laid, one in each hole, 

 burrow into the heart-wood of the 

 tree, and grow to be cylindrical, blunt- 

 ended, whitish grubs, ij inches long, 

 with short thoracic legs and a short anal 

 horn. They pupate in their burrows 

 within a cocoon made of silk and tiny FIG. 654. The pigeon-tremex, Tremex 

 Chips. The issuing winged adult gnaws columba (After Jordan and Kellogg; 



natural size.) 

 its way out through the bark. In 



some allied species (Sirex) the pupa may remain in the tree for several 

 years. Tremex is parasitized by an extraordinary ichneumon-fly, Thalessa, 

 which has a slender, flexible ovipositor, four to five inches long, with which 

 it bores into trees infested by Tremex and deposits its eggs in the Tremex- 

 burrows. The young Thalessa-grub (larva) moves along the burrow until 

 it finds a Tremex-larva, to which it attaches itself, living parasitically. (See 

 account of Thalessa, p. 483.) A small horntail sometimes abundant and 

 injurious is the European grain-cephus, Cephus pygmceus, whose larvae 

 bore into wheat-stems. The adult is f inch long, shining-black-banded and 

 spotted with yellow. It lays its eggs in tiny holes bored in the stems just 

 about the time of the forming of the heads; the larvae tunnel down through 

 the stem, reaching the lowest part of the straw about harvest-time. This 

 part is left by the reaper, and in it the larva makes a silken cocoon within 

 which it hibernates. In March or April it pupates, and the adult issues 

 in May. 



Indications of the work of certain hymenopterous insects are familiar to 

 even the most casual observers in the variously shaped "galls" that occur 

 on many kinds of trees and smaller plants, especially abundantly, however, 

 on oaks and rose-bushes. Not all galls on plants are produced by insects, 

 certain kinds of fungi giving rise to gall-like malformations on plants, nor 

 are all the insect galls produced by members of that family of small hymen- 

 opterous insects called the Cynipidae, or gall-flies. But most of the closed 

 plant-galls, and particularly those conspicuous, variously shaped, and most 

 familiar ones found abundantly on oak-trees and rose-bushes, are abnormal 

 growths due to the irritation of the plant-tissue by the minute larvae of the 



