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), ii inches long, with reddish 
head and thorax and black abdomen with yellow bands and spots along 
the sides. The females bore holes J 
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, i| 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 naturalize'^° rdan and Kellogg; 
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 
