122 



INSECTA OE HBXAPODA. 



legs, attached, to the thoracic region on its ventral surface. 

 This feature will at once separate them from aU other 

 Arthropods. Eespiration is tracheal, the openings or spiracles 

 being slit-like apertures along the sides of the abdomen, in 

 addition to two pairs on the thorax. The typical mouth of an 

 insect is described in chapter viL This is a biting mouth, a type 

 of oral structure we find in many injurious insects. Others are 

 provided with a piercing mouth, and live upon plant sap and 

 animal blood, often causing serious loss to the farmer and 

 stockbreeder. The third form of mouth, the sucking month, 



mf w 



Pig. 60.— Mouth Parts of Insect3. 



A, Biting mouth parts ; b, sucking, and o, piercing mouths : I, upper lip ; m, man- 

 dibles ; ma:, maxillEe ; mp, maxillary palpi ; Za, lower lip ; Zp, labial palpi ; a^i, an- 

 tennae. (Nicholson.) 



prevents those insects possessing it from doing any damage. 

 Those groups with this latter type of oral structure aid more or 

 less in the fertilisation of plants. It will be chiefly, then, 

 amongst the biting and piercing mouthed insects that the 

 injurious species will be found. But we must bear in mind 

 that the suctorial mouthed insects have mandibulate larvae, 

 which can do as much damage as the true mandibulate groups. 

 Many insects are distinctly beneficial by destroying other 

 noxious insects, and a few are of no inconsiderable commercial 

 value. 



