EXTREMITIES. 



>99 



re-development of a limb out of a rudiment which has meantime been latent 

 (mandibular palp of the Decapod larva, Vol. ii., p. 312, maxillipedes of the 

 Stomatopoda, Vol. ii., p. 300). A similar example is afforded among the Insecta 

 by the thoracic limbs of many Hymenoptera ; these appear as rudiments in the 

 embryo, disappear later, and reappear in the imago.* The same process will be 

 found to explain the phylogenetic appearance of the abdominal limbs of the 

 caterpillars and Tenthredinid larva : for it can hardly be doubted that the 

 Lepidoptera and the Hymenoptera, as well as all Heteromorpha, are to be 

 derived from homomorphous ancestral forms which, in the larval condition, were 

 devoid of abdominal limbs. The larval form of the caterpillars, in sj'ite of its 

 apparent resemblance to Peripatiis, must be accepted as a secondary ontogenetic 

 condition acquired in adaptation to certain conditions of life (p. 366). 



Special mention should be made of the appendages of the last abdominal 

 segment (anal or terminal segment, which in many orders of Insects, especially 

 in the lower orders (Orthoptera genuina, Ephemcridae, Odonata, Plecoptera), 

 persist throughout life as the so-called cercopoda (cerci). It must still be 

 considered doubtful, on account of the nature of the terminal segment, whether 

 we may consider these appendages as the equivalents of the other true limbs. 

 According to the observations of Cholodkowsky (No. 19), their development 

 in Blatta seems to support such a view. They here appear not only in a form 

 resembling that of the other abdominal appendages, but a process of the 

 coelomic sac which develops in the terminal segment extends into them as 

 into the other limb-rudiments. The homologue of the cerci is perhaps found 

 in the posterior extremities of the Lepidopteran caterpillar, which lie beneath 

 or near the anus, the so-called anal 2)>'olegs which, according to Grabek (Xo. 

 30). develop on the terminal segment. The three-jointed anal cerci of the 

 Tenthredinid genus Lyda and the structure known as anal spikes in other forms 

 {Nematvs, Zaddach, and Hylotoma, Geaber, No. 30) correspond to them. 

 The so-called anal prolegs of the larvae of many Tenthredinidae are, on the 

 contrary, appendages belonging to the tenth or penultimate abdominal segment. 



There is a certain relation also between the typical abdominal limb- rudiments 

 and the unjointed appendages of the ventral plate of the ninth abdominal 

 segment, known as the styli ; these are found in many Orthoptera, and persist 

 throughout life in the males. According to Cholodkowsky (No. 19), they 

 are derived in Blatta from the embryonic limb -rudiment of this segment. 

 Haase (No. 153), on the contrary, will not allow that either the appendages 

 under consideration or those small movable processes found on the abdominal 

 segments of the Thysanura (ventral stylets) have the morphological significance 

 of true limbs, but regards them merely as the equivalent of the coxal spurs 

 of Scolopendr> Ha. 



"We are here led to ask to what extent the external genital appendages, the 

 so-called gonapophyses, are to be traced back to limb-rudiments. The researches 

 of Kraepelin* and Dewitz (No. 103) have revealed that the ovipositors of the 

 Hymenoptera and the Locustidae, and the corresponding genital appendages 

 of the male in these forms, are derived from imaginal discs of the eighth and 

 ninth abdominal segments, which, when the} - first appear in the larva, closely 

 resemble those imaginal discs of the larva of Corethra, which yield the thoracic 

 limbs (p. 371). BfJTSCHLI (No. 11) and others have therefore attempted to 

 refer the gonapophyses of these forms to true abdominal limb-rudiments. In 

 support of this assumption, we might point out that these imaginal discs 



* [See footnote, p. 296.— Ed.] 



