4 The Form of Insects 



hinder ones in the male, also a pair of wings ; in the 

 female the wings are much reduced. The hind-body 

 (abdomen) is made up of eleven segments (only eight 

 of which are readily visible) j it bears no walking 

 legs, but the short, spindle-shaped appendages (cerco- 

 pods) at the tail end must probably be regarded as 

 modified limbs. 



Cell-Structure. — Before considering in detail the 

 various organs of the cockroach it is necessary to glance 

 for a moment at the minute structure of insect tissues. 

 Each visible part of any animal's body is built up by a 

 vast number of tiny, microscopical living units known 

 to the naturalist as cells, being either actually composed 

 of a multitude of cells or formed (secreted) by their 

 activity. The cell-substance is granular and semi- 

 fluid, and its essentially living constituent is a highly 

 complex chemical compound known as protoplasm. 

 Within each cell a special portion of the contents, of 

 a markedly granular nature, can be recognised as 

 forming a small nucleus. There is every reason for 

 believing that the nucleus is the seat of the cell's 

 activity. In order to study the minute structure of 

 cells, naturalists stain animal tissues with carmine and 

 other dyes before examining them under the micro- 

 scope. It is found that the nuclear substance takes 

 up these stains more readily than the general cell- 

 substance, so that the nucleus of a cell in a tissue 

 prepared for microscopic study appears intensely 

 coloured. High magnification shows, however, that 

 a special material within the nucleus, arranged in the 

 form of fine threads which are sometimes netted and 

 sometimes looped, takes up the stains much more 

 markedly than the rest of the nuclear substance. This 

 deeply-staining material is known as chromatin^ the 

 rest of the nuclear substance as achromatin. Which 

 of these two materials is especially the centre of 



