22 THE COCKROACH : 



probably exceed all domestic animals of larger size, while in 

 geographical range the five species, lapponica, germanica, 

 orientali^, americana, and australasice, are together comparable to 

 the dog or pig, which have been multiplied and transported by 

 man for his own purposes, and which cover the habitable globe. 



The Cockroach a persistent type. 



The Cockroach is historically one of the most ancient, and 

 structurally one of the most primitive, of our surviving insects. 

 Its immense antiquity is shown by the fact that so many Cock- 

 roaches have been found in the Coal Measures, where about 

 eighty species have been met with. The absence of well-defined 

 stages of growth, such as the soft-bodied larva or inactive pupa, 

 the little specialised wings and jaws, the simple structure of the 

 thorax, the jointed appendages carried on the end of the 

 abdomen, and the unconcentrated nervous system, are marks of 

 the most primitive insect-types. The order Orthoptera is un- 

 deniably the least specialised among winged insects at least, and 

 within this order none are more simple in structure, or reach 

 farther back in the geological record than the Cockroaches. 

 The wingless Thysanura are even more generalised, but their 

 geological history is illegible.* 



Life-History. 



The eggs of the Cockroach are laid sixteen together in a 

 large horny capsule. This capsule is oval, with roundish ends, 

 and has a longitudinal serrated ridge, which is uppermost while 

 in position within the body of the female. The capsule is 

 formed by the secretion of a " colleterial ' gland, poured out 

 upon the inner surface of a chamber (vulva) into which the 

 oviducts lead. The secretion is at first fluid and white, but 

 hardens and turns brown on exposure to the air. In this way a 

 sort of mould of the vulva is formed, which is hollow, and opens 

 forwards towards the outlet of the common oviduct. Eggs are 



* Brongniart has just described a Carboniferous Insect which he considers a 

 Thysanuran (Dasyleptus Lucasi), though it has but one anal appendage. See C. R. 

 Soc. Ect., France, 1885. 



