Larval Dimorphism 



The microscope shows us thirteen rings, In 

 eluding the head. This head Is small, slightly 

 horny, as Is proved by Its amber colour, and 

 bristles In front with a small number of short, 

 stiff hairs. On each of the three segments 

 of the thorax there are two long hairs, fixed 

 to the lower surface ; and there are two simi- 

 lar and still longer hairs at the end of the ter- 

 minal ring. These four pairs of bristles, three 

 in front and one behind, are the locomotory 

 organs, to which we must add the hairy edge 

 of the head and also the anal button, a sus- 

 taining base which might very well work with 

 the aid of a certain stickiness, as happens with 

 the primary larva of the Sltarls.^ We see, 

 through the transparent skin, two long air- 

 tubes running parallel to each other from the 

 first thoracic segment to the last abdominal 

 segment but one. They ought to end in two 

 pairs of breathing-holes which I have not 

 succeeded In distinguishing quite plainly. 

 Those two big respiratory vessels are char- 

 acteristic of the grubs of Flies. Their 

 mouths correspond exactly with the points at 

 which the two sets of stigmata open in the 

 Anthrax-larva in its second form. 



^A Parasitic Beetle, noted for the multiplicity of trans- 

 formations undergone by the grub. — Translator's Note. 

 103 



