SALIVARY GLAND AND ITS

SECRETION IN MOSQUITOES

M. ISWARYALAKSHMIMSC .PUBLIC HEALTH

ENTOMOLOGYVECTOR CONTROL RESEARCH

CENTRE

INTRODUCTION TO SALIVARY GLANDS • Four Pairs Of Glands Are Associated With The Oral Cavity• They Are Mandibular, Maxillary, Hypopharyngeal And Labial

Glands• Salivary Glands Are Also Known As Labial Glands• Produce Saliva• Saliva Is Mixed With Food And Ingested• In Holometabolous Insects, A Given Gland May Be Present

Only In Some Life Stage• Eg; IN LEPIDOPTERA – Mandibular Gland Occur In Larval

Stage But Not In Adults.• In Species One Or More Glands Collectively Function As

Salivary Glands

• Size, Shape, Types Of Secretion Depends Upon Food Habit• Two Basic Types – Acinar And Tubular • Acinar- In Orthoptera And Dictyoptera

• Tubular-in Diptera, Lepidoptera, Hymenoptera

• Salivarium – Region Between Labium And Hypopharynx• Saliva Stored In Salivary Reservoirs

Resemble Cluster Of Grapes

Resemble Tubes

CONTINUED…

Salivary gland of representative insects

(A) The locust (B) Adult blowfly,(C) The tobacco hornworm moth (D) leafhopper, (E) The large milkweed bug.

 • The fundamental function of saliva in insects is

lubrication of the mouthparts and lubrication of the food to assist its transport through the foregut.

• Water in the saliva also can dissolve components in the food.

• The common constituents of saliva consists of digestive enzymes, such as amylase, invertase, various proteases, and lipases.

• In some insects, salivary digestive enzymes provide the main digestive function.

• They contains microorganisms.

SALIVA OF INSECTS

SALIVARY GLAND OF MOSQUITO

The Salivary Gland Structure• Salivary glands of mosquitoes are located in the thorax, as

paired structures and are sexually dimorphic with the structure.

• Each gland consists of three lobes that are attached to a salivary duct.

• In culicine, this duct extends the length of each lobe, whereas in anophelines, it extends only part-way along the lobe.

• Female glands are larger than male glands and are differentiated into two lateral and one medial lobe.

• The lobe comprises of a secretory epithelium surrounding a duct into which saliva is released.

• The proximal regions of the lateral lobes in female and male express and secrete salivary products such as amylases and α1-4 glucosidases that are involved in sugar feeding, the medial lobe and distal-lateral lobes are associated with products involved in hematophagy such as apyrases, anticoagulants and vasodilating agents.

Structures

Salivary Reservoirs The reservoir ducts from side of the gland fuse to form the common salivary duct leading to an opening on the hypopharynx. A valve near the orifice of the common duct on the hypopharynx opens and closes as the it is raised and lowered during feeding, thus controlling release of saliva. When the insect is not feeding, the hypopharynx is in a lowered position and saliva produced by the salivary glands then backs up into the reservoirs where it is stored.Salivary PumpsMany insects with piercing-sucking mouthparts inject saliva into their food for various purposes. Often a pumping mechanism is used to inject the saliva through elongate hypodermic needle-like mouth-parts.

Saliva of mosquito Mosquitoes obtain carbohydrates from plant nectars and honeydew.

Sugar meals provide energy for somatic functions, flight and reproduction, whereas female mosquitoes are anautogenic as they require blood-meals for egg development.

Salivary gland screretions helps in lubrication, antimicrobial activity such as lysozyme, which prevents microbial growth in the sugar meals stored in the insect crop.

Feeding of blood from the host by a female mosquito takes several minutes and requires the repeated probing of host skin with its mouthparts until it locates and pierces a blood vessel.

The secretion of saliva during feeding is important for the successful location of host blood vessels and control of host haemostatic and immune responses.

The female mosquito contains salivary proteins containing at least one vasodilatory, one antiplatelet, and one anticlotting agent/molecule (many of which are antigenic) although in some cases more than one of each is present.

Salivary glands, blood feeding and immuno-modulation

During an insect bite, the salivary glands release componentsthat include antihistamines, vasodilators like tachykinin,anticoagulants like thrombin- and Fxa-directedmolecules and immunomodulators, in order to facilitateentry of inoculum containing pathogens. The salivary componentsof mosquitoes have been implicated to be of importancein transmission of pathogens.

In Aedes aegypti, the saliva was first reported to havevasodilating functions and the presence of tachykininswas reported much later. Vasodilation is of major importancein facilitating efficient blood feeding, and thusfor passage of pathogens and parasites.

• The tachykinins are peptides which act as vasodilators, with varied pharmacological functions on the central nervous system, cardiovascular and glandular tissues.

• Besides acting as a vasodilator, saliva inhibits the platelet aggregation and has anticoagulant properties.

• The vasodilators increase the diameter of blood vessels to allow greater blood flow and they have shown the presence of vasoactive peptides, sailokinin I and II in Aedes. In vitro clotting assays have revealed the presence of thrombin-directed anticoagulants in Anophelines (malaria vectors), while Culicines had Fxa-directed anticoagulants.

• Besides these observations, the anticoagulants of Culex quinquefasciatus are twice more potent than those of the Ae. Aegypti.

CNTD…

• The hematophagous insects play a crucial role in the transmission of many parasites and pathogenic organisms.

• During the short time of blood feeding, anophelines, for example, inject sporozoites along with other salivary-gland discharges. Once entered into the blood circulation, the sporozoites begin the malaria cycle in the susceptible host.

• While facilitating the ingestion of blood meal of the mosquito, the salivary glands of the insect also express defence molecules for minimizing parasite infections for its survival.

CNTD…

Salivary gland components with diverse functions

D7-like fragments of Anopheles;Expression of salivary gland-specific genes was first characterized in A. aegypti, and more recently in An. gambiae. The A. aegypti D7 gene corresponds to a 37 kDa polypeptide present in the saliva, which is encoded in five exons separated by small introns. Although the exact functions of D7 are unknown, the secretory cavities suggest their potential role in blood feeding and/or parasite transmission.

Apyrase gene product:• It is a secretory protein which hydrolyzes

ATP and ADP to AMP and Pi, and has been shown to inhibit the ADP induced platelet recruitment and aggregation.

• A few studies have revealed a relationship between sporozoite infection and the time of probing with a one-third decrease of apyrase activity in P. gallinaceum-infected A.aegypti.

• The apyrase activity is more confined to the distal regions of the female salivary glands.

FUNCTIONSThe salivary glands of mosquitoes performs the followingrole: (i) facilitate blood feeding; (ii) transmit parasites;(iii) minimize parasite infection; (iv) Produce chemical stimuli like xanthuranic acid for completion of parasite life cycle; (v) have probable receptors for recognition of sporozoites.

CONCLUSION

Salivary Glands Of Mosquitoes Perform Several Functions For Effective Survival Of The Insect, While Maintaining The ability To Harbour Pathogens And Parasites.

The Acute Structural Design And Physiology Of The Salivary Gland Makes It An Effective Organ To Perform Various Functions Conducive For Blood Feeding And Parasite Transmission.

REFERENCES The Science Of Entomology By Romoser-second Edition The Insects Structure And Function By R.F. Chapman Comprehensive Insect Physiology Biochemistry And Pharmacology –

Volume 4-regulation;digestion;nutrition;excretion Elements Of Entomology By Rajendra Singh Role Of Saliva In Blood Feeding By Arthropods-annual Review Of Entomology

32(1):463 Function And Evolution Of Mosquito Saliva –The Journal Of Biological

Chemistry Review Article-Role Of Mosquito Salivary Gland By Ravi Dhar And Nirbhay

Kumar -National Institute Of Immunology, India. Research Paper - Role Of Salivary Gland Secretions In Anopheles

Mosquito Midgut And Plasmodium Development By Robert Muhia Karanja