ON UROTHRIPS PARADOXUS. 
135 
Uzel states that four pairs of spiracles are present in the Tere¬ 
brantia , but Hinds points out that the last-named pair, the meta- 
thoracic (or meso-metathoracic) pair, are very small and in some species 
totally absent, whilst in others, where traces of stigma are to be found, 
they are vestigial, and really functionless. 
The tracheal system in Urotkrips is of the first importance, the 
species in question possessing no less than eleven pairs of stigmata, 
namely the pro-mesothoracic and meso-metathoracic pairs, and a pair 
on each of the abdominal segments one to nine. The abdominal stigmata 
in the segments one to eight are large and very distinct, and are 
somewhat remote from the edges of the segment in which they occur; 
but the stigmata on the ninth segment, though equally large, are much 
more difficult to distinguish, being placed laterally. 
Phylogenetic considerations. 
Provided that Hinds speculations as to the phytogeny of the 
Thysanoptera are correct, then Urotkrips, possessing seven-jointed 
antennæ, and single-jointed maxillary and labial palpi, etc., would 
certainly appear to be a member of one of the most recently evolved 
groups, and, following the discussions and conclusions of previous 
authors, it would at once be regarded as a more advanced type than 
any member of the Tubulifera formerly known. 
The presence of numerous pairs of stigmata in Urotkrips , in place 
of the four pairs found in the previously known Tubulifera, and in 
most, if not all, the Terebrantia , would seem to suggest a primitive 
form rather than a comparatively recently evolved type. It is a striking 
fact that in the species of Thysanoptera that have been anatomically 
examined, the main tracheal trunk on each side of the abdomen runs 
out laterally in each of the abdominal segments one to eight, precisely 
as if to open into a stigma, though the stigmata are only present in 
the first and eighth body-segments. Surely this suggests that the inter¬ 
mediate set of stigmata had only been comparatively recently lost, which 
suggestion is supported by the fact that the stigmata present in the 
Tubulifera and Terebrantia are very small compared to those in 
Urotkrips , and still further strengthened when it is remembered that 
the meso-metathoracic pair of stigmata is always present in the Tubu¬ 
lifera, but in the Terebrantia they are very small, often vestigial and 
functionless, and in some species totally absent. 
