122 INJURIOUS AND USEFUL INSECTS 



24. (PRACTICAL.) THE DEVELOPMENT OF AN 

 INSECT {Chironomus) 



Procure two or three of the egg-masses described on p. 120. 

 They are transparent, float at the margin of still water, and 

 adhere to the bank by a fine double-thread. Horse-troughs 

 or fountains in gardens are often the most convenient places 

 to search. A small spatula or penknife is useful for detaching 

 the egg-mass. 



Examine an entire egg-mass, and compare it with the 

 description already given (p. 120). Dip it into boiling water, 

 and then study the arrangement of the interlacing threads 

 which traverse the cylinder, and moor it to the bank. Heat 

 renders them sufficiently opaque to be distinguished from the 

 transparent jelly in which they are imbedded. Determine 

 the stage of development according to the table which follows. 

 Having ma4e out the chief features of any stage, replace 

 the egg-chain in water, and re-examine at intervals of three 

 hours. 



For many purposes it is convenient merely to place the 

 egg-mass in a watch-glass with a little water, and examine 

 it with a good lens. The details require the compound 

 microscope. A small piece of an egg-chain may be snipped 

 off, placed on a sunk slip and secured by a cover. A quarter- 

 inch objective is high enough for most purposes. If sections 

 are desired, the egg in the cylinder must be killed with hot 

 30 per cent, alcohol, half-saturated with corrosive sublimate. 

 They can then be gradually transferred to absolute alcohol, 

 chloroform, and melted paraffin. The sections are cut by 

 a microtome, and stained on the slide by Heidenhain's 

 haematoxylin process. It is not possible to arrange the 

 eggs in any particular position, and many of the sections 

 are therefore useless. The method is too difficult for un- 

 practised hands, and elementary students are recommended 

 to observe only the living embryo. Owing to interruption 

 for sleep and occupations, and the difficulty of obtaining 

 egg-masses of the precise age desired, several will have to 

 be procured in order to complete the history. If a good 

 site is discovered near at hand, this is not a serious difficulty. 

 The inquiry can be carried on most easily in June, July, 



