472 TROMOPTERA 



" Tromoptera. The relationship of the JVcmestrinidae, Cyrtidae and 

 " Bomhylidae is obvious, and has been noticed by the earliest writers ; it 

 " finds its confirmation in the history of the transformation of these families. 

 " They are all parasitic in their larval stages, and, as far as known, all 

 " show two stages of development of the larva, the mobile one after 

 " hatching, and the torpid, after reaching their intended host. The 

 " Cyrtidae and Ncmestrinidae, both ac beta (that is without macrochaetae), 

 " are specialized types, the former, as far as known, are parasites of 

 " spiders ; the Nemestrinidac are not universally distributed, but occur 

 " sporadically in disconnected, limited areas, far distant from each other, 

 " and characterized by a warm, dry, almost rainless, climate. (Central 

 " Asia, South Eastern Europe, some parts of Africa, of Australia and the 

 " deserts of South America.) [Compare in Berghaus's Physic. Atlas, new 

 " edit.; the map Meteorologie, N"- XI (1886) has areas tinted in the 

 " palest blue, indicating a minimum of rainfall ; those are the regions of 

 " the Nemestrinidae.'] " 



"As I have said above, the prevalence of holoptic heads in the male, 

 " connected with the power of hovering, and legs, fit principally for 

 " alighting, distinguish the TromojytGra. Dichoptic heads in the male 

 " occur only exceptionally. The number of posterior cells as amaximum 

 " is five in the Cyrtidae and Nemestrinidae and four in the Bomhylidae. 

 " But as the venation in all these families is very variable, and subject to 

 " degradation, this is not a very deep-seated character. The number of 

 " pulvilli is normally three in the Cyrtidae and Nemestrinidae, and in 

 " this, just as in the number of posterior cells, they approach the 

 " Eremoeliacta. Whether this double coincidence is an index of some 

 " hidden relationship is as yet unknown." 



"The Bomhylidae are a much more numerous family than the two just 

 " mentioned ones, spread almost universally, but always seeking dry and 

 " sunny situations. The variety of forms in this family is unique among 

 " Diptera : Bombylius, Anthrax, Lomatia are the principal types, showing 

 " a more or less distinct system of macrochaetae, especially on the sides of 

 " the thorax (Bombylius on the abdomen, concealed within a dense clothing 

 " of fur). The extremes, as to form, are the slender Systropus, entirely 

 " bare of hairs or bristles, and the heavy Toxophora with comparatively 

 " small wings, but stout legs, and showing an unusual development of 

 " stout macrochaetae on the thorax, and even a pair of ocellar bristles on 

 " the head (a unique case, I believe, among Tromoptera), almost a pedestrian 

 " among aerial Diptera ! Among all this variety of forms, however, the 

 " parasitism of the larvae is a constant character." 



" I have no hesitation in placing the Thcrevidae among the Tromoptera, 

 " but I would consider them as an ancestral form. I derive this opinion 

 " from the fact that species of this family are apparently common in New- 

 " Zealand (which, as well known, abounds in non-evoluted forms, as 

 " Dr D. Sharp calls them in his paper on N.-Z. Coleoptera). All collections 



