Water Uptake and Embryonic Development in Eggs of Chorthippus Brunneus Thunberg (Saltatoria: Acrididae)

Chorthippus brunneus has one generation a year, and overwinters in the egg stage. Richards & Waloff (1954) found a large and variable winter mortality of eggs (26–89%) during 4 years’ observations in the field: they were uncertain what factors were involved, but winter rainfall may have been important. There is, except for rare individuals (Moriarty, 1969), an obligate egg diapause that prevents blastokinesis until broken by exposure to low temperatures (Richards & Waloff, 1954). Pre-diapause development is usually completed within 14 days at 25° C. (Moriarty, 1969). Many, though not all, acridids have to absorb external water before embryonic development can be completed, and Uvarov (1966) considers this intake of water to be important in the control of diapause. The relationship between water uptake and development may therefore be involved in the field mortality of C. brunneus eggs. I have examined the relationship between water uptake and development, and the effects of desiccation and osmotic pressure.

Egg pods less than 24 hr. old were obtained from wild specimens kept in the laboratory in battery jars with sand and fresh grass. Eggs were dissected out from the pods and placed on Nylon mesh sieves (Moriarty, 1963) in 4 in. glass Petri dishes, either in air or in contact with (but not immersed in) aqueous solutions which always contained o ·1 % methyl-p-hydroxybenzoate to inhibit microbial growth. The eggs were kept at 25 ± 1° C. except when chilled at 5 ± 2°C. to break diapause (Moriarty, 1969).

Eggs were weighed singly on a Cahn electrobalance; they were handled by a fine sable brush, and dried before weighing by rolling on soft absorbent paper tissue. All weight changes are assumed to be due to water loss or uptake. Some eggs were fixed in hot Bouin fluid and then dissected to determine the stage of development.

Twelve eggs were kept in contact with distilled water at 25° C., weighed daily for 15 days, and then every other day for a further 2 weeks. After 12 weeks at 5° C. they were returned to 25° C. and weighed daily for 22 days or until hatch. Six eggs hatched, all on the fourteenth day after chilling. Time of hatch can reasonably be taken a. identical with end of embryonic development for a 14-day post-diapause period (Moriarty, 1969). There was approximately a twofold increase of weight during development, but despite the same over-all rate of development there was considerable variation in the timing of water uptake between individual eggs (Fig. 1). Typically, weight started to increase after some days, with a second, sharp, increase in weight after chilling, but the only constant factor is that weight increases at some stage during development; weight started to increase immediately after laying with some eggs, and there was not always a large increase after chilling. The embryos that failed to hatch started with a similar pattern in general, but usually weight started to drop after some stage, although this did not always occur (Fig. 2).

The eggs were dissected out from each of three pods containing 10 eggs and weighed (one was damaged during weighing). Variation in egg weight between pods was found, as usual, to be greater than that within pods (F_2.26 = 38·2* * *). Eggs from each pod were ranked in order of weight, and successive pairs were allocated randomly to, two initial treatments : kept dry or in contact with distilled water. Most of the eggs were transferred to the other treatment after various time intervals. Three eggs, one from each pod, were allocated to each time of transference. The eggs were chilled for 45 days after 2 weeks at 25° C., and were weighed after 3, 7, 14, 59, 66, 70 and 91 days.

All eggs lost weight whilst in air, and started to regain weight when they were put in contact with water. Nymphs hatched from all three of the eggs transferred to water after 3 days. One egg hatched out of the three desiccated for both 7 and 14 days, and had similar weight changes to the four eggs that did not hatch, although these embryos never reached blastokinesis. The egg that had been desiccated for 14 days hatched about 4 days later than the others. The other two groups of eggs lost weight for the whole of the desiccation period (Fig. 3), and none of the embryos reached blastokinesis. Analyses of variance on the absolute weight losses and on the angular transformation of the percentage weight losses after 3, 7 and 14 days showed no effect of egg weight or pod on water loss.

Three eggs hatched out of the five always in contact with water. None of the others developed past the diapause stage. All of them lost weight when removed from water until their weight had dropped by 53·5 ± 0·7%, when egg weight remained constant (Fig. 4). None of the eggs desiccated from the start had reached this limit, which suggests that the eggs initially in contact with water and starting to absorb it desiccate more rapidly.

To examine the influence of osmotic pressure, ten eggs, selected randomly, were placed in contact with each of four solutions: distilled water, 0·1, 0·42 and 1·0 M-NaCl. After 2 weeks at 25° C. two embryos were examined from each treatment. All stages of development up to pre-diapause were found, with no correlation with treatment or weight changes. The rate of weight change is clearly correlated with the osmotic pressure (Table 1). The eggs were then kept at 5° C. for 45 days, and then returned to 25° C. for another 38 days, when unhatched embryos were fixed and dissected out.

Half of the eggs on distilled water were transferred to 0·42 M-NaCl after chilling, which prevented water uptake (Fig. 5), but had little effect on development. Conversely, half of the eggs on 0·42 M-NaCl were transferred to distilled water after chilling, but no eggs hatched, and the embryos were at various pre-diapause stages of development, as were the embryos exposed continuously to 0·42 M-NaCl. There was little water uptake by any of these eggs (Fig. 5). The hatch from eggs on 0·1 M-NaCl was similar to that from eggs on distilled water, although water uptake was slower (Table 1). Half of the eggs on 1·0M-NaCl were transferred to distilled water after chilling. These four eggs had absorbed water for the first 10 days, but when weighed after 24 and 38 days they lost weight continuously whilst being weighed. Presumably something had happened to their water-retention mechanisms. The eggs retained on 1·0 M-NaCl after chilling also regained some weight. The embryos had developed to various pre-diapause stages. None of the embryos kept continuously on 1·0 M-NaCl had developed at all, but this is clearly a chance occurrence, because both of the embryos examined after the first 14 days showed considerable development.

The great variation between eggs in the pattern of water uptake shows that there is no precise correlation between the stage of development and the amount of water absorbed. Water absorption before diapause is clearly not essential for at least some eggs, and they can survive a considerable loss by desiccation. Loss of water by desicca-tion after diapause appears to be fatal, but prevention of further water uptake by 0·42 M-NaCl had no apparent effect. I suggest that some water uptake is essential for post-diapause development, but that it does not matter when the water is absorbed.

Browning (1967) concluded from a survey of water uptake by insect eggs : ‘Although few specific studies have been made, it is probable that in all other eggs water absorption occurs only at a particular stage of embryogenesis, although the stage at which water is absorbed differs from one species to another.’ C. brunneus is clearly an exception to this statement, and it is possible, for two reasons, that some of the species believed to support Browning’s suggestion may also show variation between individuals. Some authors obtained data on water uptake from groups of eggs, which may mask considerable individual variation, as my data show (Figs. 1, 6). Also, as Salt (1949) realized, comparisons must be made between individuals with similar rates of development; he excluded 35% of his eggs which did not develop to schedule (personal communication). This exclusion of a considerable proportion of eggs is valid, but may also conceal variability in the relationship between stage of development and water uptake. Conversely, McFarlane & Furneaux (1964) found abnormal development in up to 50% of eggs of Acheta domesticas, and inclusion of these eggs masked some changes in water content.

The mechanism of water uptake by C. brunneus is unknown. Slifer (1938) first showed water uptake to be restricted to a specialized hydropyle in Melanoplus differentialis, and such specialized areas have since been found in other species. Hartley found that ‘The whole of the egg-wall takes up water fairly readily’, in C. brunneus, and I have confirmed this. By contrast, C. parallelus has a hydrofuge egg surface except for pits at the posterior end (Hartley, 1961), but these facts give no information about absorption into the egg. The results with sodium chloride suggest that water enters the egg by osmosis, although there must also be other controlling mechanisms (Browning, 1965).

It seems unlikely that water absorption has much influence on the winter mortality of eggs in the field. The eggs are fairly resistant to desiccation, are quite flexible in the timing of water uptake, and are normally surrounded by the egg pod which pre-sumably reduces water losses.

I thank Mr C. Newbold for his assistance with this work.