4 FIRST LESSONS IN ZOOLOGY 



From which end of the egg does the young mosquito 

 emerge ? It may not be easy to find the egg-masses on 

 the pools ; in that case the wrigglers or larva; (described 

 in the next paragraph) should be sought for and brought 

 into the schoolroom in tumblers or jars containing water 

 taken from the pool in which they are found. The life- 

 history can be studied from this point on. The tumblers 

 must not be kept in places too cool or dark, or the young 

 mosquitoes will develop abnormally slowly. 



The " wrigglers " or larvae. — The newly hatched 

 mosquito bears no resemblance to the familiar winged fly 

 which we call by that name. In this first stage of its life, 

 or second stage, if we call the egg stage the first, it is 

 familiarly known as a "wriggler," but is called larva by 

 naturalists. The active young stage of any insect which 

 differs markedly from the fully developed or mature one 

 is called the larval stage. 



The larvje swim actively about. By what means do 

 they swim 1 If they cease swimming do they sink deeper 

 in the water or rise to the surface .^ Is the body of the 

 larva more or less dense than the water .'' that is, is it 

 heavier or lighter than water .'' Note that some of 



them hang quietly from the surface, and that each one 

 comes occasionally to the surface and rests there for 

 a while to breathe. Every animal has to breathe ; that 

 is, to take up oxygen from the air and to give off from its 

 body carbon dioxide (COj). There is always some air 

 mixed with or dissolved in water, and some aquatic ani- 

 mals — fishes for example — have special structures called 

 gills which enable them to take up this dissolved ox\-gen, 

 and thus to breathe under water. WvX the mosquito larva 

 has no gills, and therefore has to come occasionally to 

 the surface to breathe. 



Examine with a hand lens one of the larva.- in a watch- 

 glass of water. Distinguish the head end of the body ; 



