172 PRIMITIVE FORMS OF LIFE 



account of their fecundity an inexhaustible source of food for 

 many other animals. The appearance of Insects therefore was 

 an event of first-rate importance in Nature, and deserves to be 

 closely studied. 



There is no doubt whatever that these creatures are derived 

 from the higher Crustaceans, in which the number of segments 

 was fixed at twenty-one. In the insects themselves this number 

 is slightly reduced. At most it is nineteen in the larvae of the 

 primitive forms. It may diminish owing to the suppression or 

 transformation of the last segments of the body, but it is never 

 increased. Five pairs of appendages surround the mouth, as in 

 all Crustaceans, 1 and this number remains constant. Further- 

 more, the mandibules and the maxillae are bifurcated like the 

 claws of the Crustaceans and exhibit on a base formed by two 

 articulations, an inner branch, the endo-podite, and an outer 

 branch, the exopodite, generally transformed into a tactile organ, 

 the palp. Beyond these appendages, in most Decapod Crust- 

 aceans, come three other pairs, more or less locomotor in function, 

 assisting also in the grasping of food, the maxillipeds. Finally 

 there are five pairs of walking legs and then the abdominal appen- 

 dages. The three pairs of maxillipeds have become the thoracic 

 legs of Insects ; all the others have disappeared, except at the 

 posterior extremity of the abdomen, where there are often 

 free appendages called cerci, and others utilized in the formation 

 of the external genital apparatus. The Machilidae, Lepismidae, 

 Campodea, Japyx, and some of the Staphylinidae, 2 are the only 

 ones possessing true abdominal legs, which are repeated in the 

 Machilidae on almost all of the abdominal segments, whereas 

 among the Lepismidae they are confined to the last segments, 

 and in the other cases to the first. Everywhere else the abdomen 

 is devoid of appendages, but, on the other hand, bears as many 

 lateral respiratory orifices as it does segments. It is difficult 

 to say whether there is any connexion here between the two 

 facts, as in the case of Scorpions. In any case, the maxillipeds 

 have regained their locomotor functions and suffice for their 

 fulfilment. The three segments that bear them constitute the 

 thorax, and of these three segments the last two are provided 

 with wings. It is not very probable that these wings were 



1 These are the antenna, the labrum, the mandibules, the maxillce, and the 

 infevior labium resulting from the fusion of two maxilla, 



2 Spirachta eurymedusa. 



