The Life Cycle Of The Sand Scarab
(Excerpts from Dr Patrick S. Dale’s 1956 Thesis notes.
The only Thesis presented in New Zealand on Pericoptus truncatus.)
The work has been, for the most part, interesting and rewarding, though some sections of it were laborious. The recovery of an adult beetle during July and August requires the removal of an estimated ton of sand, and twenty beetles have been collected. In the search for pupae some fifteen tons of sand have been excavated for no reward at all. On the other hand twelve shillings and sixpence in cash has been recovered from the haunts of lovers among the dunes. Their anonymous contribution is gratefully acknowledged.
It is hoped that this study will clear the ground for more intensive work, particularly on parasitism, metamorphosis and the physiology of larval digestion. The size and relative simplicity of Pericoptus makes it an admirable subject for study, though the stages other than the egg and larva are not as readily obtainable as might be wished. Now that its biology is known, in breadth if not in depth, it would be a pity if such material were passed over. Both. as an advanced insect and as an unspecialised scarab, Pericoptus is a scientifically fortunate creation.
The Sand Scarab
Excerpts from Dr Patrick S. Dale’s 1956 Thesis, Canterbury University College, presented for Degree of Master of Science with Honours.
The life cycle occupies up to three years
For Pericoptus truncatus the cycle comprises a short egg-stage (about two months) three larval instars occupying one or two years in all, a short pupal phase of perhaps two months and a long passive adult stage of five months followed by an active phase of two or three months.
Egg are laid from early October to mid-November in various situations. They can be found most readily alongside or beneath well anchored logs and sticks on the seawall, at depths from six to eighteen inches. They occur in similar situations, though less commonly, in the beach depression, and are deposited in large numbers at depths down to two feet, among the roots of marram grass on the seaward slopes of the dunes. They have not been found in areas free of vegetable matter and are evidently always deposited close to a stable source of food for the larva. Exposed logs or logs only slightly embedded do not appear to provide a suitable environment, though occasionally occur near undecayed wood which shows no sign of previous larval attack.
The eggs are laid singly and the numbers in each region, suggest that the female commonly lays her full compliment in an area of two or three square feet.
Newly deposited egg’s are elongate ovoid or very slightly ovate with a length of about 3.7 mm and a diameter of 2.5 mm. They are creamy in colour and are covered by a secretion to which dry sand adheres strongly though wet sand does not.
Egg-laying ceases in the first half of November and hatching commences in the last week of November. The duration of the incubation period has not been accurately determined. In the field, no newly hatched larvae were found before November 25th and no unhatched eggs after- December 5 although larvae were common in the areas searched. These considerations point to an incubation period of about two months.
The newly hatched larva is about 11 mm long with a head diameter of/from 2.9 mm to 3.3 mm. It is pinkish in colour, with orange head and claws. Its body tapers sharply posteriorly with the head capsule large in proportion to body width.
Apart from the body proportions, which are the result of an almost empty hindgut, the first instar differs most noticeably from the other larval stages, in the development of the spiracles which are all sub-elliptical in outline and almost without bullae.
From seven to ten days after hatching the larvae commence feeding on decayed wood or the dead outer layer of marram grass roots, according to their situation. Up to the commencement of feeding they subsist on the yolk contained in the mid gut, and the chorion which is eaten within the first two days. It can be found in the hind gut where it is presumably digested.
Once feeding has begun the larva. changes colour from pink to a greyish purple. Fat deposits being insignificant the alimentary canal and its contents are visible throughout the length of the body, and are responsible for the colour change. The colour persists for most of the instar, giving place to a creamy colour except on the last two segments, where fat store is laid up prior to the first ecdysis.
Most larvae undergo their first ecdysis between late January and mid-March but a few persist through the winter and an occasional one can be found as late as November.
The moulted exuvium is probably eaten and digested, for though the skins are quite durable (final instar exuviae are common) neither first nor second instar skins have been found. One final instar larva was found to contain second instar mandibles in its hind gut contents.
The newly moulted second instar larvae are short (18 mm) in relation to their head width (5.2 mm). The intersegmental folds are closely concentrated and the head capsule is soft and easily depressed.
The second ecdysis takes place for many larvae in late July or early August, but almost as many remain unmoulted in December and a few can be found at all times of the year.
The final instar is of uncertain duration. It is conceivable, in view of the absence of any clearly defined size grouping in spring and summer samples, that those which begin the instar in August pupate in the following January. But in view of the fact that the pupal period appears to be confined to the early autumn, and considering the abundant presence of final instar larvae at all times of year it is most likely that they pass a further year in this phase. Fully grown (50 mm) larvae are commonly found in a torpid condition at depths of two feet or more, in autumn, winter and spring. Three of these taken in late January were kept in a jar of sand (larvae usually attack one another under such conditions) until the following December, during which time they rarely moved and fed not at all. Two died during December and the third commenced feeding before it also died. It may be that conditions in the jar were unsuitable for pupation, but it may equally well be that larvae which have made good progress in growth enter upon a quiescent phase for the remainder of the second larval year.
It is to be expected that the larval growth rate varies according to the situation in which the larva finds itself, and that within any situation – dune, beach depression or seawall – the size of larvae is as inconstant as the physical conditions governing their feeding.
Although total length is a measure of progress through an instar it is not at all reliable as an indicator of the time taken to achieve that state. Thus, when the third instar larvae of two successive seasons occur together it is impossible to decide by measurement, which were hatched in say, 1953 and have made slow growth, and which hatched in 1954 and have progressed rapidly.
A remarkable feature of third instar growth is the absence of any fully grown (50 mm) larvae between July and October. It appears to lend some support to the theory of a quiescent phase between maximum growth and pupation. If this is generally the case it could also account for the presence of large larvae in February and March when they would otherwise be expected to have pupated.
The presence of large second instar larvae in December, and of large third instar larvae in June, together with the complete absence of pupae from July till January (and probably from March till January) indicate that a two-year larval period is not uncommonly the case, but the extent of distribution of the various sizes on a scatter diagram, even for larvae under good feeding conditions as at Taumutu ( lake Ellesmere) shows that there is an immense variation in the growth rates of different larvae and a larval life of one year is by no means out of the question.
Larval surface wandering is characteristic of the species. Some emerge and start their perambulations about an hour after sunset but they are most abundant on the surface on fine nights between 10 pm and 2 am. Few remain above ground after the first light of the morning but some can be found making poor progress as late as noon unless the strength of the sun has overcome and disabled them before this time.
All instars take part, the larger larvae, second and third instar appearing first in late August and continuing the habit until the end of April. Grubs are rarely found during the winter.
The tracks are almost straight and may be as long as eighty yards for a final instar one. They show no regard for the nature of the surface or its slope, sometimes pass completely through marram clumps and avoid large obstacles, of which the larvae seem to be aware from a distance of four or five yards.
Pupae are inordinately difficult to find and I have been able to collect only five, three of which were badly damaged in collecting while the fourth metamorphosed after being kept. The beginning of pupation is therefore probably in late January and early February but the end which can only be estimated from the pupa collected on January 24 and metamorphosed on March 16 probably occurs from March to April.
Pupation takes place usually between 1 ft 6 ins and 2 ft 6 ins below the surface but occasionally at lesser depths so that shed skins are uncovered by the tide or by the wind.
The newly emerged adult is pale brown with dark brown extremities and, creamy unexposed areas. It is strongly negatively phototropic and less strongly positively geotropic. It burrows to the bottom of a jar but retires a little if exposed to light.
The first adults were found at a depth of 3 ft 6 ins on the seawall.
No adults were found at this time in the beach depression or the dunes but a few were found on other parts of the seawall and at the dune foot of the narrow beach at south Brighton.None could be found at Taumutu (Lake Ellesmere) though larvae are particularly plentiful there.
Males frequently travel three or four feet at an inch or so below the surface which cracks above the line of progress.
The extent of the “gestation” period has not been determined with any degree of accuracy.
There is some evidence for supposing that September 5 is one of the earliest dates for copulation for three males were found to be still at a depth of I ft 6 ins as late as September 18 while ten other adults of which eight were in pairs were found within a few inches of the surface on the same day. A pair near the surface were also found on September 14.
It would appear then that adults first emerge early in September which supports the above speculation regarding “gestation”.
The tracks of beetles are common on the seawall throughout September and October but no beetles were found walking on the surface before October 13 when a powerful mantle lamp was employed as a light trap. Its use on this and subsequent nights gave the following information on the habits of the adult.
Flight takes place on calm nights or in light breezes, the beetles being most active between dusk and midnight. (+ see comments below) Flights are rare when the surface sand is wet, perhaps due to the difficulty of burrowing and they appear almost to cease under any conditions.
Mr E. E. Turbott (correspondence) reports the occurrence of large numbers of flying Pericoptus before dawn on October 13, 1952 at Tauranga and Opotiki. He quotes the finder, a milkman, as saying that they appeared to have difficulty in taking off and had to wait for suitable gusts of wind. I have not been able to make dawn observations here during the flight period, and can only record a waning in numbers towards midnight. Observation continued till 1.30 am.
To what extent the light upsets normal behaviour is not known. On landing the elytra are brought together with a ripping sound and during burrowing they are often flicked one against the other to emit a series of sharp clicks. In the field the females evidently die deeply buried for skeletons are often to be found at a depth of one foot or eighteen inches, and are rare on the surface.
Many males persist till the end of November being frequently found in the vicinity of egg batches, and one was found among young larvae as late as December 6. Male skeletons are, however, common on the surface after December 1. It may be that they come to the surface at night and have not the requisite strength to burrow before being killed by the sun on the following day. A few skeletons are also found underground at a depth of about six inches.
The area under discussion (Fig. 1) comprises a strip of beach about half a mile long and 200 yards wide, forming part, of Pegasus Bay, North Canterbury. It includes the ridge of dunes fronting the beach and extends as far as mean high water spring. Henceforth this area is referred to as the “beach area”, as distinct from the “beach.” which excludes the dunes.
The coastline of Pegasus Bay lies almost north and south for the southern two-thirds of its length where sandy beaches occur. The beaches are broad and gently sloping and their upper third is not covered, by the tide except in late winter when spring high-water is reinforced by strong easterly winds. During the summer the sand of this upper zone is desiccated to a depth of about six inches and this gives rise to the peculiar form of some stretches of the beach. The prevailing winds are easterly and north-westerly so that in exposed areas of beach during the summer the dry sand is either swept up onto the dunes by the east wind or driven down the beach to the sea by the strong dry winds from the north-west.
The dunes being colonised by marram grass and sedges, retain much of the sand. they collect, the wet lower surface of the beach holds the dry sand driven onto it, and the upper third of the beach is transformed into a depression. The seaward driven sand, added to that returned by the waves, forms a broad low flat-topped ridge parallel to the coast just above mean high-water mark. Cockayne (1911) has called this ridge the “seawall” which term is used hereafter. The hollow between the seawall and the dunes is referred to as the “beach depression” (see Fig. 2). Cockayne remarks that this structure is typical of North Canterbury beaches, but the growth of pine plantations behind the dunes appears to have interrupted the north- west airflow so that the formation is now well developed only in areas such asSpencer Park, where the beach is backed by the Waimakariri lagoon and South Brighton, where the Avon-Heathcote Estuary lies inland, and the north-west wind is unimpeded except by the low dunes.
On the closely-settled parts of the coast, wind-breaks erected on the seawall have brought the main dune ridge coastwards, leaving a narrow beach where the waves approach the dune foot at normal high tides, and erode the dunes at extreme high-water.
At Spencer Park, where most of this study was carried out, winter seas occasionally flow over into the beach depression which consequently contains a fair amount of flotsam. The heavier debris such as logs and kelp becomes stranded along the summit of the seawall.
The nature of the dunes in this area shows that they are the product of the interaction of the easterly and north-westerly winds, for they do not form a continuous ridge as is commonly the case on west coast beaches, but are divided into a row of steep hummocks by deep troughs running from north-east to south-west. This is in accordance with Bagnold’s (1941) observations that dune ridges like ripple marks, lie at right angles to the winds which create them. Larvae approaching the dunes from the beach, commonly enter them by way of these transverse troughs.
The sand is everywhere well sorted. More than 88% of it is between .125 mm and .25 mm in diameter and less than 3% is finer than .125 mm. Grain size apparently has less effect on Pericoptus than does porosity, for larvae are rarely found in sand whose porosity is below 42% but they occur in coarse sand at Taumutu – Lake Ellesmere, (83% between 1 mm and .25 mm) where the porosity is about 45%, the sand being poorly sorted. But neither grain size nor porosity are responsible for limiting p. truncatus to the beach area, and the sand behind the main dune ridge, and as far as two miles inland at Wainoni,(both of these situations are inhabited solely by P. punctatus)is virtually identical in character with that on the beach.
Nor is salt content of the surface sand of much significance as a specific barrier. On the seawall surface salinity (down to 3 inches) varies from .05 to 1.4 grams per kilogram of dry sand, in accordance with the amount of rain or salt-spray to which it is subjected. In areas recently covered by waves, on the seawall and in the beach depression it may be as high as 1.52 grams, in the dunes and behind them a figure of .05 is fairly consistently maintained. (The method used for measuring salinity is not accurate except for comparative purposes when such small quantities are being considered and .05 probably indicates conditions virtually free from salt). Yet P. truncatus occurs in abundance in all these areas, (P. truncatus appears to be intolerant of salt conditions on the beach) and at various depths on the seawall, where a similar saline fluctuation may be observed (see Fig. 3).
These variations are likewise reflections of rainfall and tidal encroachment except where, at depths greater than four feet, a rapid rise in the degree of saturation is accompanied by a fall in salinity suggesting the influence of ground-water seepage.
The requisite temperature records have not been kept to establish whether or not temperature extremes or rates of change, constitute a barrier to P. truncatus but in view of its narrower tolerance as compared with P. punctatus, (see Natural History) it is possible that this is the case. The difference of temperature range between the beach and its “hinterland” must be considerable for it is not uncommon for the dune surface to be frozen to a depth of two inches while the open beach remains above zero temperature.
Sand is the sediment in which Pericoptus usually lives. P. punctatusis occasionally found in sandy loam or riverbed silt, but clean sand is by far the commonest substrate for the genus and most sizeable bodies of sand throughout New Zealand, seem to contain one species or another.
Sand is normally defined as material whose mean diameter lies between 0.01 mm and 1.0 mm but a more useful definition due to Bagnold (1941) describes terrestrial sand as the material which is small enough to be lifted bodily by the prevailing wind velocity, but which is too large to remain suspended in the airstream. Hence sand at Pegasus Bay is much finer than that on the exposed beach at ‘I’aumutu (Lake Ellesmere) largely by virtue of the difference in wind velocity.
The definition draws attention to another property of wind-blown sand. It is unique among sediments in being self-accumulatory, so that although its particles are individually unstable, and although the whole body of sand may shift, it tends to remain as a body without being dispersed. It thus constitutes a constant, if fluid environment.
The sand of Canterbury beaches is almost entirely of quartz origin (Cockayne 1911) and is not significantly subject to chemical decay, being the product of physical contact between particles, (Marshall 1929) which are later sorted by water and by wind. It is not therefore subject to much internal chemical or physical variation in any given area.
…..the temperature of the surface layer increases markedly during the day and it is not uncommon in summer for such temperatures on Canterbury beaches to exceed 60°C. Consequently, although final instar larva of P. truncatus has been known to survive thirty-six hours of fine February weather on dry concrete, it is rare for a larva to survive a single day on dry sand whatever the state of the sky, and those which fail to burrow within a few hours of sunrise appear to lack the strength to burrow later, or to move to more suitable surrounding’s.
Poor conductivity has the opposite effect on sand below the surface so that at depths greater than six inches the sand on most parts of the coast never becomes completely dried out, and is cushioned to a remarkable degree against sudden changes of temperature.
Poor conductivity is chiefly due to the angular and irregular shape of the grains, which inhibits their close packing. The volume of enclosed spaces is never one third of the total volume of a sample of the grain sizes under consideration. These voids are responsible for other important physical characteristics. They restrict capillary action to a height of 24 cms above the water table (Fig. 7) and sand down to four feet deep on the seawall is not normally invaded by capillary water. On the other hand seepage is extremely slow, an inch of water requiring thirty-six hours to penetrate compacted dry sand to a depth of 30 cms, A wave advancing and receding over slightly damp sand saturates it to a depth of only 5 cms. After a rainfall during May, of 2 .23 inches in twenty-four hours dry sand occurred on dune crests 20 cms below the surface.
Thus between depths of eighteen inches and three feet, the region in which pupae and immature adults occur, the water content of sand is susceptible to only slight and gradual variations.
Wet sand has other peculiarities. It exhibits the phenomenon of “bulging” (Terzaghi and Peck 1948 p.118) by which the surface tension between grains of slightly damp sand is such that it inhibits their falling into a compact mass. Thus when the surface sand of the beach is dried out by strong sunshine the expansion of the interstitial vapour causes the top six inches or so to increase in bulk, while the moisture remaining in it prohibits the grains from collapsing. Bulged sand is a common feature of New Zealand beaches after heavy rain followed by strong sunshine in summer, or during low water of summer spring tides.
When sand dries completely or becomes saturated these surface tension forces are absent and the grains fall into a compact mass. Thus saturation, or the moistening of dry sand by rain, causes a temporary compact surface which Pericoptus larvae find difficult to penetrate. Its porosity ,may be as low as 41% (cf. Table I).
In the vicinity of rotting timber however, these extreme effects -are somehow offset (perhaps due to timber forming a sponge-like reservoir) so that bulked or partly bulked sand usually occurs around and beneath driftwood on the seawall, constituting “oases” in the inhospitable intervening surface. To a less extent marram grass clumps play a similar part. This may be an explanation of the attraction of larvae to solid objects on the surface in times of water shortage or surfeit.
The beach area has many properties unshared by other substrata, properties which impinge at many points on the biology of the animals which live there.
I have pleasure in acknowledging the advice and encouragement of Professor Percival and Dr Pilgrim of the Canterbury College Biology Department. I am particularly indebted to Mr B. B. Given of the Entomological Research Station, Nelson, for information and advice on a wide range of subjects including speciation and distribution of the genus. To my fellow student Mr W. C. Clark I extend thanks for assistance and advice on photographic work and on general field practice. Of both Mr Given’s and Mr Clark’s worldly wisdom and kindness I have received an ample share.
I am also grateful to the entomologists of the New Zealand museums and to the good people both here and overseas who have answered my questions, directed me to sources of information and sent me reprints of their papers.
Thanks are also due to my wife for financial assistance and helpful criticism of the draft manuscript.
NB: These notes of Dr Patrick Dale’s (of which only a very small portion of the original thesis is reproduced here) may be of crucial importance for those wishing to replicate the beach environment more naturally and hence achieve better results in the laboratory or for home experiments.
It is a 130 page document comprising an immense number of dedicated hours in the field shifting many tons of sand, and is a quite remarkable testament to the perseverance of someone who obviously has a great love of Nature, and in particular Pericoptus.
NB: The Thesis in its entirety —
The Sand Scarab, PERICOPTUS
Thesis presented for degree of Master of Science with Honours
– may be viewed through any Central Library throughout New Zealand but may not be borrowed to take out.