78 INSECT PHYSIOLOGY 



be going on throughout metamorphosis. At times it must happen 

 that these two processes will exactly balance in respect to a particular 

 substance over a given period, and then chemical analysis will reveal 

 no change at all. It is clear, therefore, that such analyses of the whole 

 animal are of rather limited interest ; or rather, are impossible to in- 

 terpret intelligently, until they have been correlated with the histo- 

 logical sequence of events. Conversely, intricate cytoplasmic changes 

 are described in the fat body and blood cells during metamorphosis, 

 the chemical significance of which is incompletely known. Ribonu- 

 cleic acid, proteins, fats, and carbohydrates are being synthesized, 

 particularly by the fat body cells. In what form they are liberated for 

 supplying the growing imaginal tissues is uncertain. 



Some chemical products of insects 



Insects produce a wide range of organic compounds for many of 

 which they have found biological uses, and some of which have 

 become articles of commerce. Prominent among these are the fibrous 

 protein 'silk', from the commercial silkworm; the 'waxes' of the 

 honey-bee and other insects, which are closely related to the water- 

 proofing waxes of the cuticle (p. 4) ; the mixture of resinous sub- 

 stance, hard wax, and pigment which forms the iac' of commerce, 

 the product of dermal glands in various Coccidae. 



Hymenoptera Aculeata produce 'venoms' which range from the 

 formic acid of Camponotine ants, to the toxic protein of bee-venom. 

 Lepidoptera, Coleoptera, and Heteroptera produce 'defensive sec- 

 retions' of the utmost variety. Many contain quinones, or unsatur- 

 ated aldehydes; salicylaldehyde, hydrocyanic acid, and histamine 

 may be secreted. Caterpillars, notably of Papilonidae, will take 

 up and discharge the poisonous compounds in certain plants (Arist- 

 lochiaceae, Umbellifera, &c). Other substances, many of them not 

 yet identified chemically, serve as 'pheromones' and act as chemical 

 messengers in relation with other individuals of the same or different 

 species. They may serve as 'warning substances', 'sexual attractants', 

 'aphrodisiac scents', means of 'social communication', and so forth. 

 One of the best known of the last group is the 'queen substance' 

 secreted by the mandibular glands of the queen bee, which has been 

 identified as 9-oxodecanoic acid and is used by the workers to 



