124 MAUD D. HAVILAND 
Teleas, do not describe the later stages of the larvae, and say 
nothing about the tracheal system. Keilin and Thompson (12) 
describe nine pairs of spiracles ina Dryinid larva, parasitic m 
Typhloecyba (Homoptera). The relative positions are not 
determined, but from the figure it seems as if the meso-, or 
possibly the metathorax, bears no spiracles. 
I can find no other account of the tracheal system of the 
Proctotrypoidea, and until we have more knowledge of the 
hymenopterous larvae which live upon their hosts as external 
parasites, we cannot tell how far the characters observed 
indicate true phylogenetic relationships, or are merely 
secondary adaptations. Moreover, it is unwise to compare a 
highly modified imternal parasite, such as Platygaster, 
with the more generalized external forms; though in this 
connexion it may be significant that the third stage larvae 
of Platygaster and Humicrosoma have a certain 
resemblance to the early larva of Ly gocerus. 
The differences are not confined to the tracheal system. 
Marchal describes ten ganglia in the nerve cord, and three 
Malpighian tubes, in Synopeas rhanis. Keilin and 
Thompson observed thirteen ganglia, and no Malpighian tubes, 
in the Dryinid that they studied. This diversity of structure 
indicates either that little reliance can be placed on larval 
characters, which are often adaptive, or that the Proctotry- 
poidea as at present understood are, im some respects, an 
arbitrary group. 
Economic STATUS. 
From an economic standpomt Lygocsrus must be 
regarded as an injurious insect. Parasitisation by Braconidae 
is an important natural check upon the increase of plant-lice ; 
and this Proctotrypid, like the hyperparasitic Chalcidae and 
Cynipidae, is an enemy of the beneficial Aphidius. Unless, 
as seems improbable, it confines its attacks to a single species, 
it must destroy considerable numbers of Aphidiidae.t A p hi- 
1 Kieffer records that L. testaceimanus has been reared from 
arose aphis (?Macrosiphum rosae) (18, p. 51). 
