The role of cuticle growth in the feeding process of ticks.

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Abstract
SUMMARY.: Those ticks which are adapted for ingesting exceptionally large blood meals possess the ability to secrete new cuticle during the period of attachment to the host. This mechanism is described in the female of Ixodes ricinus. The period of engorgement can be divided into two phases. (a) During the first, which lasts for seven days, the body weight rises gradually from 2 mg. to about 50 mg. In this period of active cuticle synthesis the endocuticle increases in thickness from 50μ to 105μ. The moderate increase in surface area is due to the molecular growth of the cuticle and not to its distension by the gut contents. Blood is ingested continuously but slowly during this phase. As, however, digestion and assimilation are proceeding actively, the lumen of the gut remains almost empty, (b) During the second phase, which occupies one day or less, cuticle deposition ceases and a large volume of blood is rapidly ingested. The body weight rises abruptly to over 200 mg. The cuticle is stretched and returns to an overall thickness of about 50μ. During the deposition of cuticle the epidermis becomes greatly hypertrophied. The nuclei enlarge, one or more nucleoli appear and the cytoplasm becomes heavily charged with ribonucleie acid and alkaline phosphatase. The processes of growth and stretching greatly modify the cuticle structure. The pore canals, numbering about one hundred and thirty per epidermal cell, become more widely separated during the growth phase. The mechanical stresses produced by stretching cause the pore canals to be pulled still further apart, and to assume an oblique or sinuous course. All species of the family Ixodidae, and usually all instars of the same species, grow their cuticles during engorgement. (The larvae and nymphs of Haemaphysalis inermis are probable exceptions.) The Ixodidae are therefore slow feeders. Among the Argasidae. the larvae commonly possess the growth mechanism and therefore feed slowly. The nymphs and adults accommodate the smaller blood meal simply by stretching the preformed cuticle. They therefore feed rapidly. (The second nymphal instar of Otobius megnini is probably an exception.) The large size of the blood meal in the Ixodidae has permitted a reduction in the instar number without any corresponding sacrifice in fecundity. This may be advantageous because host‐finding is also thereby reduced. However, the slow process of cuticle growth entails the lengthy carriage of the tick by the host and this may cause dispersal of unfavourable environments. The suppression of feeding and the cuticle growth phase in the larvae of Ornithodoros moubata and O. savignyi suggests that under certain conditions selection has in fact operated in the reverse direction.

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