4o8 



PHYLUM ARTHROPODA 



As to the pedigree of insects, the wingless CoUembola 

 and Thysanura are doubtless primitive. In Protopteron, 

 for instance, there are appendages on the first four seg- 

 ments of the abdomen, and the genital apertures are paired. 

 Similarly, Acerentomon is a little blind creature, without 

 antennae, without cerci, without stigmata, with suctorial 

 mouth-parts, with eleven abdominal segments, with a 

 peculiar anal segment, with an unpaired genital aperture 

 on the eleventh urosternite. For Acerentomon, Acerentulus, 

 and Eosentomon (with stigmata) the special order Protura 

 has been proposed. These and similar primitive forms 

 lead us back to some of the less specialised Myriopods 

 {e.g. Scolopendrella), back further to the level represented 



Fig. 227. — Acerentomon, a very primitive insect. 



H., Head ; THA, TH.i, TH.^, terga of thoracic segments ; /, 2, j, the 

 thoracic legs; .4.1, A. 2, A.^, A A, abdominal appendages; P. A., 

 eighth abdominal tergum ; ^., genital aperture ; .4 P.P., post-anal 

 appendix. 



by Peripatus, which helps to link the Tracheate to the 

 Annelid series. 



But though the primitive wingless insects, the simple 

 types of Myriopods, and Peripatus, represent ascending 

 steps in evolution, what the actual path has been we do 

 not know. 



Insects and Disease 



Many insects play a very important part in the spread 

 of disease. The most serious cases are those of insects 

 which act as hosts as well as carriers of disease-producing 

 parasites. The blood-sucking Diptera are the most 

 important in this respect. At the head of the list stand 

 the mosquitoes or gnats of the family CuHcidae. 



A large number of mosquitoes belonging to the genus 



