The Specific Location of Zinc in Insect Mandibles

Mandibles were dissected from insects freshly killed by freezing at – 20 °C and any adhering soft tissue was removed. The mandibles were rinsed briefly in distilled water and then in methanol to remove surface debris held on to the mandibles by cuticular lipids. The mandibles were then blotted and mounted on a copper or aluminium stub with silver paint. An X-ray microanalysis of the uncoated mandible was made in the scanning mode of a JEOL model 100S electron microscope fitted with a Kevex detector and a Link analyser. Analyses were made with an accelerating voltage of 40 kV, specimen tilt of 35° and a spot size of approximately 70 nm. Analyses were counted for 100s on an energy range of 0–20 k eV but spectra were recorded from 0–10 k eV only as no information is lost by using this narrower energy range. X-ray maps were made by monitoring for energy in the range 8580–8660 eV corresponding to the zinc Kα emission peak.

A minimum of six analyses of both left and right mandibles were made of all the species listed in Table 1. As shown for the locust (Fig. 1), all mandibles contain silicon, phosphorus, sulphur, chlorine, potassium and calcium. Such elements may be expected in a cuticular matrix normally in contact with epidermal cells. (In all spectra there were emissions corresponding to silver, copper and sometimes aluminium ; these arise from the specimen stubs, the silver paint used to stick specimens on to stubs and the lenses of the microscope.) The locust mandibles also contain zinc in a sufficiently large amount to show both the Kα and the Kβ peaks (Fig. 1). No zinc was found in any other piece of locust cuticle examined. These were the prothorax, femur ribs, tibial shaft, tibial spines, tarsal claws, semilunar processes and the rest of the tibio-femoral joint, abdominal tergites and sternites and the ovipositor valves of the female. Mandibles from other species were also found to contain zinc and/or manganese (Table 1).

When present in the mandibles, zinc and/or manganese are exclusively located in the cutting edges. The location of zinc was found by X-ray mapping (Chandler, 1977) as shown for the minima caste worker of the ant, Atta cephalotes (Fig. 2). In all cases the occurrence of zinc is confined to the cutting edges (the teeth in the ant) and it is not found in the sheared cuticle or the supporting cuticle of the locust or any other species examined. Manganese is present in lesser quantity and therefore its location had to be found by a large number of analyses of the morphologically distinct areas of the mandibles. Again it is restricted to the cutting areas only. Most insects that were examined were herbivores and most of these contain zinc or manganese in the mandibular incisors (Table 1). The mandibles of herbivorous nereids also contain zinc (Bryan & Gibbs, 1979). Neither zinc nor manganese was found in the mandibles of the carnivorous beetle or the omnivorous cockroaches examined (Table 1).

There is some indication that the presence of zinc or manganese in the mandibles of the herbivorous insects is related to their taxonomic grouping. Thus manganese was not found in the orthopterans, phasmids or lepidopterans ; and among the hymenopterans, manganese was always found together with zinc (Table 1). The picture is somewhat confused as the only coleopterans to contain zinc are woodboring larvae. However the presence of these elements might also be related to feeding habits, the composition and mechanical properties of the foodstuff or the feeding mechanism employed.

Only ash contents of whole insects or whole cuticles have been reported previously and these are always less than 0· 3 % of the dry weight (Richards, 1951). The amount of zinc present in the whole mandible of the Heliothis larva and the locust (Table 2) shows that the zinc content, alone, of the mandible is several percent of the dry weight. In effect the actual zinc content of the cutting edges may be some 3–4 times higher still as the values in Table 2 are for the whole mandible and the zinc is highly localized.

The observation that the cutting edges of the locust mandibles are harder than the rest of the cuticle (Hillerton et al. 1982) may be explained, in part, by the present results as being due to the presence of large amounts of zinc. We may assume that the analogous cutting edges of other herbivorous insects that also contain zinc and/or manganese are similarly hardened, although the mandibles are often much too small to measure, and that these elements may be responsible for the hardening.

We have no evidence that zinc or manganese is involved in mineralization (e.g. Runham et al. 1969). Zinc is always found in the presence of a large amount of chloride, but similar amounts of chloride are present in other areas of the mandible also. The zinc may increase the density of secondary bonding within the material by forming complexes with proteins and phenols, rather like the complexes formed by zinc, protein and melanins in the vertebrate eye (Bowness & Morton, 1953), and thus increase the density and fracture toughness of the mandibular cuticle.

This work was financed by a grant from the Agricultural Research Council. We are indebted to Mr A. Jenkins for instruction and help in the use of the X-ray microanalyser and to various other colleagues for the supply of live insects.