49^ THE INVERTEBRATA 



thorax and first eight abdominal segments. The mandibles are typi- 

 cally strong and dentate; the maxillae are stumpy and consist of a 

 cardo, stipes and single maxillary lobe with a two- or three-jointed 

 palp : the labium has a large mentum, a prementum bearing a median 

 spinneret and small two-jointed palps. 



The thorax bears three pairs of legs, and the abdomen five pairs of 

 prolegs on segments 3-6 and 10. Such prolegs are different from the 

 typical insect limbs, being conical and retractile with hooks on the 

 apex (Fig. 344 C). In many families there are less than five pairs of 

 prolegs, and in Micropteryx there are eight pairs. 



These larvae feed almost exclusively on flowering plants (excep- 

 tions being the Lycaenid caterpillars which are carnivorous, feeding 

 on aphides or entering ants' nests and devouring the larvae). Their 

 digestive enzymes are modified for dealing with plant tissues. 



The pupa, which is disclosed after the last larval moult, is usually 

 protected by a cocoon previously prepared by the larva. In the case 

 of Tortrix moths the cocoon is largely composed of leaves drawn to- 

 gether by silk strands. In others, e.g. the silkworm moth, Bombyx 

 mori, it is composed of silk and from it the silk of commerce is pre- 

 pared. Agglutinated wood particles form a hard cocoon in the puss 

 moth, Dicranura. In Pieris, the pupa is naked and attached to the 

 substratum by the hooked caudal extremity, the cremaster^ and by a 

 delicate girdle of silk about its middle. In the most primitive forms 

 (e.g. Micropterygidae) the pupae are free, their segments are free to 

 move and the appendages are not fused to the body. Obtect pupae, 

 in which only few segments are movable and the appendages are 

 fused to the sides of the body, are most common, e.g. Platyhedra 

 (Fig. 338 B). Free or incompletely free pupae often emerge from the 

 cocoon before the emergence of the adult. 



Lepidoptera are almost invariably harmful in the larval stage, few 

 plants being free from their attacks, and some of the world's most 

 serious insect pests, such as the cotton bollworm, Platyhedra gossy- 

 piella^ and the gypsy moth, Porthetria dispar, are included in this 

 order. 



The order is divided into two suborders. In suborder I, Homo- 

 neura, the fore and hind wings have venations which are almost 

 identical. To this primitive feature may be added that of the included 

 family Micropterygidae whose mouth parts are mandibulate and the 

 structure of whose maxillae and labium are easily comparable with 

 those of the cockroach. 



The ghost moths or swifts (Heptalidae) are also included in this 

 suborder. These nocturnal insects have vestigial mouth parts and 

 short antennae. Their jugate type of wing coupling has already been 

 described. In certain species^t.g. Hepialus humuli, the female searches 



