THE PROBLEMS OF TRANSFORMATION 253 



with some special need for protection in its life-conditions (as 

 has been illustrated in a previous discussion on larvae in 

 relation to their surroundings, p. 232) like the conspicuous 

 caterpillars of " eggars ", " tussocks ", and many butterflies. 



There is, however, one feature distinctive of caterpillars that 

 may, with much probability, be regarded as a primitive sur- 

 vival : the presence of paired limbs on many of the abdominal 

 segments. There may be seven or eight pairs of these prolegs 

 on the abdomen of a saw-fly caterpillar, and it is well known 

 that five pairs are usual on the larva of a moth or butterfly. 

 These prolegs are clearly adaptive structures, especially suited 

 to the caterpillar's habit of crawling along twigs or the edges 

 of leaves ; but there is no reason to doubt that the embryonic 

 rudiments on the abdominal segments from which they grow, 

 are appendicular in nature, and serial with the thoracic legs 

 like the evanescent limb-rudiments that appear for awhile on 

 certain abdominal segments of some beetle-embryos as well as on 

 those of ants. The truly appendicular nature of prolegs is also 

 strongly supported by the fact that in the moss-eating larvae of 

 the Micropterygidae (Fig. 122) these limbs may be jointed like 

 the thoracic legs. And this group of little moths is the 

 most primitive section of all the Lepidoptera, as shown by the 

 undifferentiated wing-nervuration, the presence of mandibles 

 in the imago and in the pupa, and of a lacinia in the maxilla 

 of the adult. A caterpillar-like larva, also with jointed prolegs, 

 is found among the Mecoptera, that small, antique order of 

 which the scorpion-flies (Panorpidac] form the principal family. 



No coleopteran larva has any trace of abdominal prolegs, 

 but it will be remembered that in those highly interesting 

 insects, the mayflies (Ephemeroptera, p. 93-5), the aquatic larva 

 and nymph has a series of paired abdominal limbs modified 

 into tracheal gills. Mayflies are insects which show a remark- 

 able commingling of primitive with specialized characters in 

 the adult, and their larvae are suggestive of bristle-tails 

 (Thysanura) adapted for life under water ; for they have 

 crustacean mandibles, relatively large and conspicuous maxil- 

 lulae, long tail-feelers (cerci),a.nd a median tail-process, while, 

 as we have just noticed, they possess on the abdominal 

 segments a series of pairs of gills which are modified limbs 

 structures comparable to the abdominal limbs of a machilid 



