Characteristics of Insecta. cxiii 



be viviparous, and the larva in one sub-order (the so-called Pupi- 

 parae), is so far developed, by means of the nutriment furnished 

 to it by a gland opening- within the maternal oviducal canal, as 

 to be nearly ready to enter upon the stage of chrysalis or pupa 

 when set free from the mother's body. The form which an insect 

 has on leaving the egg always differs more or less from that which 

 it possesses when adult, and capable, as only full-grown insects are, 

 of reproducing its kind by sexual genesis. The larvae of apterous 

 insects, such as certain of the OrtJioptera and Hemiptera, differ 

 from the adult, irrespectively of the undeveloped state of the gene- 

 rative organs, only in such points as size, the nimiber of joints in 

 the antennae, and the number of facets in the corneae. It is only 

 in the quantitative increase accruing to these two sets of organs 

 of special sense that the adult after the entire number of its moults 

 comes to differ from the larva in the way of heteronomy. A 

 greater degree of heteronomy is attained to in the families of the 

 two orders specified which are endowed with wings, by the super- 

 addition of those organs, and by the concomitant greater differentia- 

 tion of the thoracic from the abdominal region of the body. Insects 

 which go through either of these two series of metamorphosis are 

 called ' ametabolous.' A third kind of metamorphosis is that in 

 which the adult insect, whilst gaining certain organs which the 

 larva does not possess, such as wings, loses certain others, which 

 the larva does possess, such as the provisional structures making up 

 the ' mask' of the Lihellulidae. Such insects are called ' Hemimeta- 

 bolous,' and in them the heteronomy of the various regions of the 

 body is always well pronounced. Insects, finally, which when adult do 

 not only differ very markedly from their larval forms both by general 

 heteronomy and by the conformation of their particular organs, but 

 also attain to this condition after going through a period of quies- 

 cence known as the ' pupa' or ' chrysalis' stage, preparatory to their 

 final moult and the assumption of the adult condition, are called 

 ' Holometabolous.' A period of quiescence as ' pupae,' in addition to 

 the period of quiescence as ova, gives the Holometabolous orders of 

 Insects an advantage as regards their distribution over the colder 

 regions of the earth, relatively to the orders the pupae of. which are 

 active; and, therefore though certain small Poduridae resist cold 

 well, it is amongst the Holometabola that we find a nearer approach 

 made to cosmopolitanism than is usual elsewhere amongst Insects. 



h 



