128 INSECT TRANSFORMATION 



addition to numerous minute spines scattered over the thin, 

 pale cuticle. After two moults the Bibio larva (Fig. 72 A) 

 is fully grown, having reached a length of 10 mm. (f inch), the 

 head being of the same rounded shape as before, but relatively 

 shorter and narrower compared with the body. The body- 

 cuticle is tough and brown, drawn out into long conical pro- 

 cesses arranged in transverse rows across the segments, and 

 bearing numerous closely-set scale-like spines, many of which 

 have rows of blunt teeth. As in the first stage the body has 

 twelve segments, of which the foremost (prothorax) is much 

 longer than the others and imperfectly divided by a shallow 

 transverse furrow. This larva is remarkable for the number 

 of its spiracles ; there are ten pairs of these, a pair on every 

 segment except the mesothorax and the ninth abdominal 

 segment. The spiracles (Fig. 72 A sp) stand out laterally 

 from the body on short cylindrical processes, except those of 

 the tail-segment (Fig. 72 D) which are much larger than the 

 others and situated on the dorsal aspect ; the spiracles on the 

 prothorax, about half the size of these hindmost, are twice as 

 large as the rest, and it is noteworthy that in the newly-hatched 

 larva only the tail-spiracles are recognizable. Turning to the 

 head-appendages we find that the feelers are very small, the 

 mandible (Fig. 72 B) stout and strong with blunt teeth, the 

 maxilla (Fig. 72 C) with short palp, and broad spinose lobe, and 

 the labium consisting of a jointed plate without definite palps 

 or lobes. At the extreme hinder end of the abdomen two 

 thin-walled conical processes can be protruded by blood 

 pressure ; these serve as tail-prolegs. Such " black-fly " grubs 

 live underground, feeding sometimes on decaying organic 

 material and sometimes on fresh plant tissues ; they have been 

 found in numbers devouring potato tubers (Plate I). The 

 nervous system of the Bibio larva consists of a brain and 

 sub-cesophageal ganglion in the head, and a ventral chain 

 of three thoracic and eight abdominal ganglia. 



Another more familiar root-feeding larva the " leather- 

 jacket " grub of a crane-fly (or " Daddy-long-legs) " (Tipula) 1 

 may serve as our next type from among the Diptera. The 

 well-known flies (Fig. 73 a) lay their eggs in the soil in summer, 



1 J. Rennie : " On the Biology and Economic Significance of Tipula 

 paludosa ". Ann. Appl. BioL, III. 1917. 



