Fundamentals of Entomology (2+1)

Digestive glands

A)  Salivary glands: In Cockroach a pair of labial glands acts as salivary gland where the salivary ducts open into salivarium. In caterpillars mandibular glands are modified to secrete saliva, where the salivary glands are modified for silk production.
• Functions of saliva:
  1. To moisten and to dissolve food
  2. To lubricate mouthparts
  3. To add flavour to gustatory receptors
  4. In cockroach the saliva contains amylase for the digestion of starch.
  5. In honey bee saliva contains invertase for sucrose digestion
  6. In Jassid saliva contains lipase and protease for lipids and protein digestion. Jassid saliva also contains toxins which produces tissue necrosis and phytotoxemia on the plant parts.
  7. In plant bug saliva contains pectinase which helps in stylet penetration and extra intestinal digestion.
  8. In mosquito, saliva contains anticoagulin which prevents blood clotting.
  9. In gall producing midges saliva contains Indole Acetic Acid (IAA).
  10. In disease transmitting ectors the saliva paves way for the entry of pathogens.

B) Hepatic caecae and midgut epithelial cells: It secretes most of the digestive juices. Two types of cells were involved in the enzyme secretion.
Holocrine : Epithelial cells disintegrate in the process of enzyme secretion.
Merocrine : Enzyme secretion occurs without cell break down.

Digestive enzymes

Insect Group	Enzyme		Substrate
Phytophagous larvae	Amylase		Starch
Maltase		Maltose
Invertase		Sucrose
Omnivorous insects	Protease		Protein
Lipase		Lipid
Nectar feeders	Invertase		Sucrose
Wood boring insects and Termites	Cellulase		Cellulose
Meat eating maggots	Collagenase		Collagen and elastin
Bird lice	Keratinase		Keratin

C. Microbes in digestion: In the insect body few cells were housing symbiotic microorganisms called as mycetocyte. These mycetocytes aggregate to form an organ called mycetome.
i) Flagellate protozoa - It produces cellulase for cellulose digestion in termites and wood cockroach.
ii) Bacteria - It helps in wax digestion in wax moth.
iii) Bed bug and cockroach obtain vitamin and aminoacids from microbes.

These microbes were transmitted between individuals through food exchange (mouth to mouth feeding) called trophallaxis and through egg called as transovarial transmission. In plant bug and ant lion grub partial digestion occurs in the host body prior to food ingestion called as extra intestinal digestion. In most of the insects digestion occurs in mid gut.