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INVERTEBRATE LEBENSSPUREN OF FLOODPLAINS: THEIR MORPHOLOGY, ORIGIN AND PALEOECOLOGICAL SIGNIFICANCE

Brett C. Ratcliffe University of Nebraska-Lincoln, [email protected]

J. A. Fagerstrom University of Nebraska-Lincoln

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Ratcliffe, Brett C. and Fagerstrom, J. A., "INVERTEBRATE LEBENSSPUREN OF HOLOCENE FLOODPLAINS: THEIR MORPHOLOGY, ORIGIN AND PALEOECOLOGICAL SIGNIFICANCE" (1980). Papers in Entomology. 136. https://digitalcommons.unl.edu/entomologypapers/136

This Article is brought to you for free and open access by the Museum, University of Nebraska State at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Papers in Entomology by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. JOURNALOF PALEONTOLOGY,V. 54, NO. 3, P. 614-630, 1 PL., 4 TEXT-FIGS., MAY 1980

INVERTEBRATE LEBENSSPUREN OF HOLOCENE FLOODPLAINS: THEIR MORPHOLOGY, ORIGIN AND PALEOECOLOGICAL SIGNIFICANCE

BRETT C. RATCLIFFE AND J. A. FAGERSTROM Division of Entomology, University of Nebraska State Museum; Department of Geology, University of Nebraska, Lincoln 68588

ABSTMCT-Although rocks of floodplain origin are volumetrically important, they contain relatively few trace fossils; both abundance and diversity are low. Conversely, Holocene floodplain sediments locally contain abundant and diverse lebensspuren mostly produced by , spiders, nematodes, annelids and molluscs. At least 8 orders and 3 1 families include species that burrow in floodplain sediments and yet none of their lebensspuren are unique to this environment. Taxonomically dissimilar insects produce morphologically similar lebensspuren, and the same species, or individual, may produce very dissimilar lebensspuren. Thus, identification of tracemakers for rocks of floodplain origin is as difficult as for marine rocks. Trace fossil form genera morpholog- ically similar to Holocene floodplain lebensspuren include Skolithos, Cylindricum, Sabellarifex, Ma- canopsis, Planolites, Palaeophycus, Sinusites, Cochlichnus, Amphorichnus and possibly also Scolicia; many previous authors have regarded these as more typical of marine environments than of flood- plains.

INTRODUCTION are more dispersed in floodplains than in DESPITEthe abundance and variety of Holo- stream channel deposits, vertebrate paleonto- cene floodplain lebensspuren, these biogenic logists have generally expended less effort col- structures are rare in the fossil record and also lecting from ancient floodplains than from are virtually unstudied in comparison to those channels (pers. commun., R. M. Hunt, Jr. and from marine and freshwater (mostly lacus- M. R. Voorhies, 1976). The rigors of living in trine) environments. Furthermore, the record the unstable substrates of active channels of vertebrate produced terrestrial trace fossils, (both erosional and depositional) virtually pre- especially tracks, is considerably better docu- cludes discovery of trace fossils in association mented (Voorhies, 1975) than the record of in- with body fossils from channel deposits. How- vertebrate traces. ever, such associations would be expected The purposes of the present paper are two- from deposits of floodplain origin. The fact fold: 1) to describe the morphology and origin that trace fossils are usually destroyed during of some common Holocene floodplain lebens- transportation may provide useful evidence spuren, especially those produced by insects for in situ accumulation when found in asso- and 2) to evaluate the paleoecological signifi- ciation with bones and thus aid in the recon- cance of Holocene lebensspuren for the rec- struction of fossil communities (sensu Fager- ognition of ancient floodplain environments. strom, 1964). Achievement of these goals will partially fill Published research on Holocene and fossil gaps in the most recent compendia on trace invertebrate lebensspuren of nonmarine origin fossils (Frey et al., 1975, p. xi; Hantzschel, is indeed meager. Because a majority of Ho- 1975). Although the present report is confined locene floodplain lebensspuren are produced to floodplain lebensspuren, many of the same by insects, the chief researchers have been forms are known to occur in sediments and entomologists and, not uncommonly, their rocks deposited in other environments, both work on burrow morphology has been inci- marine and upland dunes (Ahlbrandt et al., dental to their prime efforts dealing with body 1978). morphology, systematics, ecology, or econom- Except for glacial deposits, rocks of flood- ic aspects of insects. The chief compilation of plain origin are volumetrically the most im- Holocene nonmarine invertebrate (insects, spi- portant of any nonmarine sedimentary envi- ders, crustaceans, "worms," etc.) lebensspuren ronment. They locally contain an abundant is Chamberlain (1975), but the major emphasis mammalian fossil record, but because bones in this work is on aquatic, rather than flood-

Copyright 1980, The Society of Economic Paleontologists and Mineralogists FLOODPLAIN LEBENSSPUREN 615 plain environments. The coverage by Cham- pods are found in the substrate either berlain is, however, remarkably diverse and ovipositing, pupating, resting, feeding, con- includes several forms from near-shore ter- structing dwelling or brood chambers, or seek- restrial environments. ing temporary refuge from the weather and Data relating to body morphology that can natural enemies. be inferred from fossil lebensspuren may be Some species burrow into sediment of vari- severely limited. The disparity between body able texture that may be saturated or dry, and burrow morphology becomes even more loose or compacted; others have been reported apparent when one realizes that similar look- burrowing into solid rock (Stephen et al., ing burrows are made by an array of different 1969). The terrain may be flat to vertical; bur- invertebrates, or that differently shaped bur- rows in vertical banks normally remain open rows may be made by a single "maker" or dif- longer than those on horizontal surfaces be- ferent ontogenetic stage of a single "maker" cause they are less likely to be filled with de- (Osgood, 1975). Moreover, any given burrow bris by wind or rain. Plant cover varies, and may be occupied by parasites, predators, or large, dense roots may inhibit some digging even a burrow "thief" which has supplanted forms. Species that forage in the substrate pre- the original "maker." Not uncommonly, or- fer organically rich soil whereas nesting ganisms may inhabit natural cavities in the species prefer soils of low organic content. substrate which may be incorrectly considered Burrow depths range from horizontal tunnels to have been excavated by the occupant just beneath the surface of the soil (semi-en- (Palmer, 1928). dostratal) to shafts 2.7 m deep in some Scar- abaeidae (Howden, 1955). TERMINOLOGY Osgood (1972) found that the amount of or- Frey (1973) has described and defined a ganic matter in the 0, horizon was the most large number of special terms used in ichnol- important soil characteristic in determining ogy but did not include the following which whether or not a particular area may be ex- are essential to the present report and also are pected to have solitary bee nests. Areas with not included in Torre-Bueno (1962): high levels of organic matter in the 02horizon cell-a subterranean cavity, usually at the end had significantly fewer nests. He also noted that nesting sites for these bees have sparse to of a shaft or tunnel, which is generally ovoid and larger than the diameter of the shaft or moderate plant growth on soils that are drained and with good surface flow. Rau tunnel to which it is connected. Used for (1925) suggested that the most important fac- depositing eggs, pupating, or turning around. (Text-figs. 3c, 4b, 4c, 4e-g, 4j, 41, tor for burrowing by an andrenid bee was the 4m; P1. 1, figs. 2, 3.) amount of rainfall and the resulting level of the water table. Smith and Hein (197 1) noticed chimney-an above ground structure, made of that the concentration of staphylinid mud and clay, which is vertical, cylindrical tunneling activity varied depending on grain and usually open at the apex. Generally made to keep out rain, predators, or para- size and cohesion of the sediment while Willis sites from burrow entrance. and Roth (1962) demonstrated that soil mois- runway-a surface groove or trench used re- ture determined whether or not burrowing peatedly as a pathway. would occur by a species of (Hemip- tera). According to Sakagami and Michener (1962), the shafts of halictid bees usually ex- ECOLOGY OF BURROWING tend below the level of the cells, possibly serv- INSECTS AND SPIDERS ing as a drain for excess rainwater or to pro- Insects and spiders burrow in the soil for a vide communication with more humid soil number of reasons. A complete or partial sub- levels in times of drought. terranean existence has adaptive value be- Silvey (1936), in his study of burrowing cause the sediment is an environment where freshwater beach insects, found that wind temperature is fairly constant, moisture is often influenced the distribution of burrowing higher, light is absent, predaceous and para- insects, especially if it was prolonged or strong sitic pressures are often reduced, and food re- which resulted in forcible scattering. He noted sources may be more abundant. Many arthro- that wind may also influence food supply, sta- BRETT C. RATCLIFFE AA'D J. A. FAGERSTROM FLOODPLAIN LEBENSSPUREN 617 bility of the beaches, and water content of the DIVERSITY OF BURROWING surface sand. Moreover, wave action caused SPIDERS AND INSECTS occupancy of the narrow inner beach to be Spiders and insects that burrow in Holocene more hazardous than other areas of the shore. floodplain sediments are locally very abundant Stephen et al. (1969) mentioned that the (Stanley and Fagerstrom, 1974, Fig. 13B) and presence of available water often influences diverse and are capable of producing lebens- the selection of a site by chimney forming an- spuren of considerable variety. Of lesser im- thophorid bees nesting in hard, dry soils. These portance are crustaceans, annelids, nematodes species transport drops of water which are and molluscs. used to moisten and soften hard, dry surfaces The following is a summary of the 8 orders so that excavation of the shaft and construc- and 31 families of extant spiders and insects tion of the chimney can proceed. that contain burrowing species in floodplain Evans and Eberhard (1970) noted that com- sediments. Their burrows are potentially ca- parisons of the gross features of the nests and pable of preservation as trace fossils. Selected nesting behavior of various is related to examples of insects, spiders and their burrows the evolution of wasps and the origin of var- are illustrated in Text-figs. 1-4. For further ious aspects of their complex behavior as well information, Borror, DeLong and Triplehorn as that of their relatives, the ants and the bees. (1976) give a general survey of the Class In- They concluded that one of the major adap- sects and their relatives. tations achieved by wasps in relatively recent geologic time was the making of deep, com- CLASSARACHNIDA: ORDER ARANEIDA plex nests in the soil which permitted survival (spiders) during adverse environmental conditions or heavy parasite pressure. In a similar case, CTENIZIDAE(trap door spiders), ANTRO- Sakagami and Michener (1962) concluded that DIAETIDAE (antrodiaetids), THERAPHOSIDAE primitive halictid bees made nests with the (), and LYCOSIDAE(wolf spiders) cells dispersed in the sediment around the (Text-fig . la). Spider tunnels (Text-fig . lb) shaft. They believe the evolution of cell ar- may be simple or branched, and some have rangement has proceeded toward increased side chambers which are separated from the concentration of the cells, and that this clus- main burrow by hinged doors. Most spider tering permits greater economy of labor which burrows are lined with silk which may help to may have selective advantages resulting in re- inhibit collapse of the walls. duction and disappearance of the lateral tun- nels leading to the cells. Stephen et al. (1969), CLASSINSECTA in discussing bees, noted that although nest ORDERORTHOPTERA (grasshoppers, crickets, architecture is a direct expression of behavior, roaches, etc.) it is probably impossible at this time to con- GRYLLOTALPIDAE(mole crickets) (Text-fig. struct a phylogenetically significant outline of Id): burrow in moist sand or mud, frequently architectural types; various groups of bees near bodies of water. The generally horizontal (and other insects) have evolved structural burrow (Text-fig. le-f) is usually just beneath patterns for their nests along parallel lines, but the surface and may branch repeatedly. Frey "progress" has not always proceeded towards and Howard (1969) and Hanley, Steidtmann increasing complexity. and Toots (1971) illustrated probable mole

TEXT-FIG.1-Habitus views of burrowing spider and insects with examples of associated burrows; total range of burrow morphology not shown. Measurements of spider and insects are approximate body length. a, Lycosa sp. (Araneida:Lycosidae),5-25 mm. b, burrow pattern common to lycosid wolf spider and cicada nymph. c, Tibicen sp. nymph (Homoptera:Cicadidae), 15-25 mm. d, Gryllotalpa sp. (Or- thoptera:Gryllotalpidae), 25-35 mm. e, f, dorsal and lateral aspect respectively of near-surface burrow of mole cricket. g, Tridactylus sp. (:Tridactylidae), 2-10 mm. h,i, pygmy mole cricket burrows. BRETT C. RATCLZFFE AND J. A. FAGERSTROM cricket (not mole beetle) burrows, and Cham- terranean root feeding areas just prior to adult berlain (1975) discussed and illustrated their transformation. The shafts are made by pre- tunnels near water. The forelegs of mole crick- liminary scraping away of soil from within the ets are adapted for scraping and pushing aside burrow followed by compacting (also from moist sand or soil as the insect moves forward within the burrow). as it feeds. TRIDACTYLIDAE(pygmy mole crickets) ORDERCOLEOPTERA () (Text-fig. lg): burrow in the loose, saturated CICINDELIDAE(tiger beetles) (Text-fig. 20: sand or mud near streams and'lakes. Burrows the adults dig burrows in which to spend the (Text-figs. lh-i) are of varying depths and con- night, escape inclement or hot weather, and figurations. Chamberlain (1975) presented a to overwinter. The predatory larvae are mor- good account of the pygmy mole crickets and phologically adapted for burrow life; larval illustrated their burrows. burrows (Text-figs. 2g-h) may be vertical to right-angled, straight or curved, and from a ORDERDERMAPTERA () few cm to 1.25 m in depth. Balduf (1935) com- ,LABIIDAE, LABIDIURIDAE, piled data from several sources on habits and :many species frequently lay burrows, and Criddle (1907) provided data on their eggs in a burrow in the soil; the female burrow depths in different soils and described guards the eggs in the burrow. The method of the construction of hibernating burrows of burrow construction is probably undescribed. several species. Wallis (1961) described the mode of burrowing, and Shelford (1908) dis- ORDERHEMIPTERA (true bugs) cussed tiger beetles and their burrows. SALDIDAE(shore bugs) (Text-fig. 2a): most CARABIDAE(ground beetles) (Text-fig. 3a): species inhabit the damp soils adjacent to bod- both the larvae and adults of many species of ies of water. Many species burrow, but the this large family burrow in floodplain habitats burrows (Text-figs. 2b-c) have been poorly creating a great variety of burrow configura- described. tions (Text-figs. 3b-e). Kirk (1972-1975 b) and GELASTOCORIDAE(toad bugs): some species Silvey (1936) provided detailed observations dig burrows in the sand, loose soil, or mud on carabid burrows. near rivers, lakes and ponds (Hungerford, LIMNEBIIDAE(minute moss beetles): many 1914). of these very small beetles tunnel in the damp CYDNIDAE(burrower bugs) (Text-fig. 2d): sand at the water's edge or make use of tunnels usually found under stones or logs, in sand, or excavated by carabids, staphylinids, and other in molds near the roots of grass tufts. Willis shore-dwelling insects (Leech and Chandler, and Roth (1962) discussed the characteristics 1956). of the cells (Text-fig. 2e), burrowing, and en- STAPHYLINIDAE(rove beetles) (Text-fig. 30: vironmental factors influencing cydnid behav- the larvae and adults of many species burrow ior. Cydnid cells seem to normally lack an ac- on beaches and sand bars; the burrows vary cess shaft. greatly in configuration (Text-figs. 3g-i). Smith and Hein (1971) discussed and illustrat- ORDERHOMOPTERA (cicadas, leafhoppers, ed the burrows of Bledius spp. Zur Strassen and their kin) (1975) indicated that some Bledius spp. feed CICADIDAE(cicadas): mature nymphs (Text- on algae growing on single sand grains in the fig. lc) construct a vertical emergence shaft damp layer just beneath the surface, and that (25-50 cm long) (Text-fig. lb) from their sub- these algae are collected by the adults and

TEXT-FIG.2-Habitus views of burrowing insects with examples of associated burrows; total range of burrow morphology not shown. Measurement of insects is approximate body length. a, Pentocera sp. (:Saldidae), 5-8 mm. b,c, shore bug burrow patterns. d, Pangaeus sp. (Hemiptera:Cydnidae), 3-10 mm. e, burrower bug cell; disruption of the adjacent sedimentary laminae is the result of collapse into the loose-walled access burrow. f, sp. (Coleoptera:Cicindelidae), 7-15 mm. g,h, larval burrows of tiger beetles (after Shelford, 1908); burrow in g with pupation chamber. FLOODPLAIN LEBENSSPUREN 620 BRETT C. RATCLIFFE AND J. A. FAGERSTROM FLOODPLAIN LEBENSSPUREN 62 1 stored along the walls of galleries or in special cell may be composed of hard clay containing chambers where the larvae will find them. tiny grains of sand with larger stones encrust- HETEROCERIDAE(variegated mud-loving ing the exterior while the inner surface is beetles) (Text-fig. 3j): the larvae and adults smooth (Spradbery, 1973). Spradbery also not- live in tunnels in the sand and mud along the ed that many mason wasps use excavated soil shores of streams and lakes. The galleries to build temporary chimneys of variable de- (Text-fig. 3k) are horizontal, just below the sign and length. surface, meandering, and often branched. POMPILIDAE(digger wasps) (Text-fig. 4d): Chamberlain (1975) noted that the walls of the the adults of many species excavate and pro- burrow were "striated", but we have not ob- vision ground burrows for their young. The served this. Silvey (1936) described and illus- morphology of the burrows (Text-figs. 4e-g) trated the burrows of larval and adult hetero- is exceedingly variable among the species, but cerids. Tunnels are made by pushing through it is generally a simple, oblique tube with a the substrate. terminal cell. Evans et al. (1953), Evans and SCARABAEIDAE(scarabs) (Text-fig. 4a): the Yoshimoto (1962), Powell (1958), Rau and members of several subfamilies of scarabs Rau (1918), and Williams (1956) observed and form feeding burrows and often elaborate described the burrowing behavior and burrow breeding burrows (Text-figs. 4b-c) for their morphology of several genera. young. Halffter and Matthews (1966) com- SPHECIDAE(solitary wasps) (Text-fig. 4h): piled a detailed account of these structures for members of most of the subfamilies nest in the the dung beetles of the subfamily Scaraba- soil and provision their nests (Text-figs. 4e-g) einae. with various captured insects which serve as food for the larvae. Burrowing by sphecids has ORDERMECOPTERA (scorpionflies) been described by Cazier and Mortenson BOREIDAE(winter scorpionflies): larvae live (1964, 1965a-c), Evans (1958, 1965, 1966a, b), in subterranean shafts to depths of about 15 Evans and Eberhard (1970), and Rau and Rau cm (N. D. Penny, pers. commun., 1977) where (1918). they are phytophagous on the rhizoids of moss- (bees): burrowing members of this es. Maximum burrow diameters are about 3 superfamily belong to the families COLLETI- mm. DAE (plasterer bees), ANDRENIDAE(Text-fig. 4i) (mining bees), HALICTIDAE(also mining ORDERHYMENOPTERA (ants, wasps, bees) bees), MELITTIDAE(melittid bees), MEGA- FORMICIDAE(ants): the ground nests of CHILIDAE (leafcutting bees), and ANTHOPHOR- these familiar insects range from small and IDAE (digger and cuckoo bees). Bohart (1952) simple to very large and complex. Most species and Stephen et al. (1969) gave the following have a system of runways, shafts and tunnels, data for the bees in general: the soil nests but others may have only a simple burrow and (Text-figs. 4j-m) of solitary bees are usually some of these are very near bodies of water. branched and contain brood cells; the main VESPIDAE(paper wasps): some of these so- burrow may be vertical, meandering, a down- cial wasps construct nests of hexagonal paper ward spiral, or oblique, and it is frequently cells in the ground (e.g. Paravespula; Rath- lined with fine particles of soil which are prob- mayer, 1975). ably tapped into place by the pygidium. Many EUMENIDAE(mason wasps): some species species line the burrow or cells with a wax-like form cells with access shafts; the walls of the or varnish-like waterproof secretion while oth-

TEXT-FIG.3-Habitus views of burrowing insects with examples of associated burrows; total range of burrow morphology not shown. Measurement of insects is approximate body length. a, Bembidion sp. (Coleoptera:Carabidae), 2-7 mm (other carabids to 25 mm). b-e, subterranean carabid burrows: one dorsal view (d, with entrance hole) and three lateral views. f, Bledius sp. (Coleoptera:Staphylinidae), 3-10 mm. g-i, burrows. j, Heterocerms sp. (Coleoptera:Heteroceridae),2-6 mm. k, dorsal aspect of near-surface heterocerid burrow system. BRETT C. RATCLIFFE AND J. A. FAGERSTROM FLOODPLAIN LEBENSSPUREN 623 ers (Megachilidae) line the cells with plant organisms (Text-figs. la-c; 4c-e) and morpho- materials brought in from the field. Burrow logically dissimilar burrows and trails may be depths range from 2-90 cm. Most burrowing produced by the same individual organism bees construct lateral tunnels from the main (Text-figs. If-g; 2a; 2g; 2e-f; 3a-d; 4a-b; 4f- burrow with one to several cells present on g). Furthermore, none of the Holocene le- each lateral; the generally ovoid cells may be bensspuren with which we are familiar is vertical or horizontal, single, in linear se- unique to floodplain environments. quence, or clustered. The nest (burrow), or portions of it, are plugged after the cells have BIOTURBATION been provisioned and capped. Some species The above discussion has emphasized the plug only the area adjacent to the cell while making of discrete burrows (shafts, tunnels, others completely backfill the laterals, and still cells, etc.) having moderately firm walls and others plug the nest entrance. a discernable relationship to the layering of the At the supra-generic level, some differences enclosing sediment. However, the activities of in architectural plans are discernable (i.e., all numerous floodplain invertebrates (especially halictid nests have a wider entrance tunnel insects) displace the sediment in ways that de- than branch tunnels), but in some families (i.e. stroy or greatly modify the original layering the Megachilidae) the diversity of nest types (bioturbation) over extensive areas in some defies classification. The burrows and burrow- cases (Smith and Hein, 1971). ing behavior have been described by many Insects may push, pull, lick or otherwise authors; chief among these have been Bohart manipulate the sediment, either at or below (1964), LaBerge and Isakson (1963), LaBerge the surface, as they move over or through the and Ribble (1966a, b), Linsley, MacSwain and sediment. Much of this activity involves feed- Smith (1952, 1955), Sakagami and Michener ing on organic matter between or on the (1962), Stephen (1966), and Stephen, Bohart grains; however, unlike numerous types of and Torchio (1969). "worms", insects do not actually ingest the Study of Holocene floodplain lebensspuren sediment nor do they secrete a mucus lining in in Nebraska indicates that the sampling of their burrows. Thus, the walls of insect bur- burrow morphologies in Text-figs. 1-4 (based rows are commonly less distinct (lack an al- primarily on the entomological literature) is teration "halo") than those of "worms" and very incomplete. The burrows and trails other . Insect feeding may be con- shown in P1. 1, figs. 1-3, made by unknown ducted in a random manner resulting in varied , are morphologically quite different degrees of sediment disruption (Text-fig. 2; P1. from those in Text-figs. 1-4 and none can be 1, figs. 1, 4) which produces a gradation of confidently related to any of the burrowing structures from discreet trails and burrows to spiders or insects described above. Thus, it is moderate bioturbation in discontinuous layers clearly evident that the same general frustra- to almost complete homogenization of thin tions experienced by marine ichnologists in sedimentary units (Pl. 1, fig. 5; see also Stanley attempting to ascribe most trace fossils to par- and Fagerstrom, 1974, fig. 4). ticular Holocene and ancient tracemakers will also plague ichnologists studying floodplain le- HOW INSECTS BURROW bensspuren, i.e. similar burrow or trail forms Knowledge of the manner in which crawling may be produced by taxonomically dissimilar and burrowing Holocene organisms produce

TEXT-FIG.4-Habitus views of burrowing insects with examples of associated burrows; total range of burrow morphology not shown. Measurement of insects is approximate body length. a, Canthon sp. (Coleoptera:Scarabaeidae), 4-15 mm. b,c, dung beetle burrows. d, sp. (:Pompilidae), 5-15 mm (other pompilids to 30 mm). e-g, burrow patterns common to pompilid spider wasps and sphecid solitary wasps. h, Sphecius sp. (Hymenoptera:Sphecidae), 20-40 mm. i, Andrena sp. (Hymenoptera:Andrenidae),5-10 mm (other bees to 15 mm). j-m, burrows of solitary bees (Apoidea). BRETT C. RATCLIFFE AND J. A. FAGERSTROM preservable structures in soft sediments is vital modification, these terms could be expanded to a proper understanding of their possible to include most other insects: trace fossil analogs (Hallam, 1975). Evans Rakers scrape the soil beneath the body us- (1966a) stated that behavior is what an ing the front legs which are curved toward the does with its structure and structure is what midline so that the spines comprising the tarsal an animal uses to behave. The structure (body comb are directed downward. The front legs morphology) of most burrowing insects is, in move alternately (Pompilidae) or synchronous- some way, adapted for digging. The mandi- ly (most Sphecidae), and the sediment is eject- bles are often used for scraping, breaking up ed backwards under the abdomen and out of the sediment and for dragging pebbles, etc., the nest. Nests are generally oblique as this and are usually more robust than in non-dig- method does not work for vertical burrows. ging forms; species nesting in compacted clays The observations by Moore (1906), von Len- tend to have broader mandibles than those gerken (1916), and one of us (J.A.F.) would nesting in sand. The forelegs of digging insects place the Cicindelidae in this category also. (Text-figs. Id, lg, 3f, 4a) frequently have Pushers back out of the burrow pushing shovel-like, expanded areas or more spines soil behind them with the aid of the well-de- and hairs for handling excavated materials. veloped pygidium which acts like a ram. Some Lastly, the pygidium is usually well developed Carabidae and Scarabaeidae do this. and flat in those species using it for pushing Pullers gathered the loosened soil into a or tapping the sediment. ball-like lump between the head and front legs Numerous insects (e.g. heterocerid beetles) and is pulled out as the insect backs out of the form tunnels by simply pushing through the nest. The pile of soil may be deposited im- soil and compacting it so that a tunnel remains mediately at the surface or dragged a few cm after they have passed; there is no actual ex- from the nest. Most burrows are oblique as in cavation or removal of soil from a chamber. the rakers. This method of tunneling is readily seen in the Carriers excavate in much the same way as saturated sand near the edges of lakes and do pullers except that the removed soil is ac- rivers. Most of the burrowing Hymenoptera tually taken some distance from the nest either and some of the Coleoptera actually remove by walking or flying, thus leaving no evidence the sediment from their nests or burrows and of digging at the nest site. pile it near the entrance. "It should not be assumed that the four Olberg (1956; see also Evans, 1966b and types of digging are mutually exclusive; for Evans and Eberhard, 1970) provided the fol- example, Tachytes mergus starts the nest as a lowing widely accepted terms to typify the 'raker,' then becomes a 'puller' when it reaches major modes of digging by wasps; with little damper sand; Bembix spp. may be pullers

FIG. 1-Holocene surficial trails (grooves) and semi-endostratal tunnels in mud; potential Sinusites and Planolites respectively. "Makers" unknown. Length of bar scale 10 cm. Vertical view. Santa Paula Creek near Santa Paula, California. 2-Unbranched, inclined endostratal shafts and possible insect cell () in recently collapsed vertical face of floodplain sand. "Makers" unknown. Length of scale 11 cm. Middle Loup River near Mullen, Nebraska. 3-Inclined and vertical endostratal shafts and solitary (a) and clustered (b) insect (?) cells in recently collapsed vertical face of floodplain sand; open entrance to one shaft near upper right corner; cf. Text-figs. 4j-m. Length of bar scale 4 cm. Middle Loup River near Mullen, Nebraska. $-Partial bioturbation (by insects?) of ripple-marked Holocene floodplain sand. Diameter of coin 24 mm. Vertical view. Elkhorn River near West Point, Nebraska. 5-Complete bioturbation by insects of loose, damp, surficial Holocene floodplain sand. Length of bar scale 2 cm. Vertical view. Elkhorn River near Scribner, Nebraska. 6-Shallow (barely endostratal) tunnel system; same area as P1. 1, fig. 5 except that loose surficial sand has been removed to reveal tunnels; cf. Text-figs. le, 3d, 3k. Length of bar scale 2 cm. Vertical view. FLOODPLAIN LEBENSSPUREN 625 626 BRETT C. RATCLIFFE AND J. A. FAGERSTROM

TABLE1-Broad categories of Holocene floodplain lebensspuren as possible analogs of ancient trace fossil genera

Potential trace Abbreviated description Selected Holocene fossil genus* of Holocene lebensspuren examples (and examples) 1. Endostratal (exichnia) unbranched, cylindrical, Text-figs. lb, 4k; Stanley Skolithos, Cylindricum, vertical shafts (dwellings; shelters) up to at least and Fagerstrom, 1974, Sabellarifex; Stanley and 20 cm. deep and lacking terminal cell. fig. 13B. Fagerstrom, 1974, figs. 3, 9B, 13A. 2. Endostratal (exichnia) unbranched, cylindrical, Macanopsis, vertical to steeply inclined shafts (dwellings; Amphorichnus shelters) up to at least 20 cm. deep and with terminal cell. 3. Endostratal (exichnia) cylindrical shafts Undescribed and (dwellings; shelters) with variously complex side unnamed. passages and cells. 4. Shallow endostratal to semi-endostratal Text-figs. li, 2c, 3k. Planolites; Stanley and (endichnia or exichnia) unbranched, cylindrical Fagerstrom, 1974, Fig. to ellipsoidal, unpacked tunnels. 8 (center). Hanley et al., 1971, fig. 3. 5. Shallow endostratal to semi-endostratal Text-figs. le, If, 3d; Palaeophycus; Stanley and (endichnia or exichnia) branched, cylindrical to Hanley et al., 1971, Fagerstrom, 1974, fig. 8 ellipsoidal, unpacked tunnels. fig. 5. (upper left). 6. Surficial (exogenic) unbranched trails (hypichnal PI. 1, fig. 1; Pryor, 1967, Sinusites; Seilacher, 1963, and epichnal grooves and ridges) of varied sizes figs. 3-8. p. 82-83. Cochlichnus; and shapes. Moussa, 1970. Scolicia?; Turner, 1978. * For generic descriptions see Hantzschel, 1975, p. W57-W108.

when handling small stones, although typical- push-up, or mound. In active or freshly ly rakers par excellence; Ammophila spp., al- worked nests a is evident. but it is though carriers, often do a certain amount of often rapidly destroyed by wind or iain. Con- raking when opening or clearing the nest. Nor spicuous tumuli often provide recognition should it be assumed that all examples of one landmarks for parasites and predators, espe- type behave identically. For example, al- cially bombyliid flies which flick their eggs into though most rakers build up a pile of soil at the entrance (Linsley, 1958). Kirk (1974) ob- the nest entrance, others dig in such a way served that adult carabids extrude excavated that the soil particles are sprayed over a wide soil from their burrows by "pushing" and that area" (Evans, 1966b). According to Evans and if the soil was moist, the extruded soil occa- Eberhard (1970), combinations of scraping sionally formed masses 1-2 cm long that re- and pushing are characteristic of more gener- tained the shape of the burrow entrance. Oth- alized digger wasps while pulling probably er burrowers (notably Eumenidae, some evolved as a mechanism for handling more Masarinae [Vespidae], some Anthophoridae) compacted soil, and carrying as a modification construct an earthen chimney at the nest en- involving total removal of soil particles which trance which is thought to keep out water or resulted in greater concealment of the nest. parasites and predators. Virtually all burrows are of a simple tubular Silvey (1936) provided a key to the burrows morphology due to the twisting and spiraling of eight species of adults and five species of movements of the "maker." In those insects larvae in two families of beetles (carabids and that plow through the substrate, burrows are heterocerids) based on branching, depth, di- more apt to be slightly wider than high; rarely ameter, and orientation. Minkiewicz (1933) there may be scrape marks on the inside of the proposed names for 11 types of terrestrial nests tunnel but these cannot generally be attributed in the Sphecidae, and Malyshev (1921, 1935) to any particular body part. suggested a complete classification of the nest The entrance to many burrows is character- types of bees; other classifications of bee nests ized by a small to moderate conical pile of ex- were given by Iwata (1942) and Stephen et al. cavated soil variously described as a tumulus, (1969). These nest classifications are largely FLOODPLAIN LEBENSSPUREN descriptive and seem to be little used in the that may be made by insects, nematodes (Wal- current literature, possibly because the archi- lace, 1968), annelids, molluscs (Pryor, 1967), tectural plans of nests are almost endless in etc. (Category 5, Table 1). Although the prob- their variety and do not lend themselves to ability of such trails becoming trace fossils is convenient classification. low, they do occur as hypichnal and epichnal grooves and ridges (Moussa, 1970). In the Ho- PALEOECOLOGICAL SIGNIFICANCE locene examples we have seen, the trails have The lebensspuren produced by the various no regular pattern or arrangement (they wan- activities of insects, spiders and other inver- der aimlessly), are generally unbranched and tebrates described above can be grouped into are good analogs for such trace fossils as Sin- six broad categories (Table 1) for the purpose usites (Seilacher, 1963, p. 82-83) or possibly of interpreting them as possible Holocene an- Scolicia. alogs for trace fossil form genera. Selection of As noted in Hantzschel (1975, p. W108, the potential trace fossil genera in Table 1 was 117), the precise morphological differences be- guided by the following three assumptions: 1) tween several trace fossil genera characterized the host sediment (matrix) is sufficiently dif- as cylindrical, vertical tubes of various sizes ferent from the cast sediment that the wall of have been debated by ichnologists for over a the fossil is clearly recognizable so it can be century. Our purpose here is to point out that inferred that the original trail or burrow was floodplain sediments also contain varied bur- open and then passively filled by sediment rows of this general form (Category l, Table from above rather than by collapse of the trail 1) and, if preserved in the fossil record, could or burrow wall, 2) the lebensspuren have been be included in the genera Skolithos, Cylindri- formed with minimal disruption of lamination cum and ~abellarifez.Previous authors (e.g. in the matrix and therefore lack spreiten and Seilacher, 1963, 1967; Alpert, 1974) have re- 3) the walls of the lebensspuren are smooth garded these genera as the dwelling structures and lack "scratch marks." of suspension feeders (e. g. phoronids, anne- Most of the genera listed in Table 1 are lids. atremates) that lived in intertidal to shal- widely assumed to be characteristic of, or even low subtidal environments. However, the ver- confined to, rocks of marine origin. However, tical tubes in floodplains are made by spiders our research clearly indicates that for rocks of and insects as shelters (sensu Stanley and Fa- Pennsylvanian age (when the body fossil re- gerstrom, 1974, p. 75) for preying, resting, cord of large insects begins) or younger, this pupating, etc. and thus clearly indicate that assumption may be invalid. Thus, future in- both the environmental and ethological inter- terpretations of both water depth and salinity pretation of these genera in Pennsylvanian and based on these taxa should give strong consid- younger rocks must be modified to include eration to the possibility that the rocks con- floodplains and uplands inhabited by a great taining them are of floodplain or even upland variety of terrestrial organisms. We know of origin. no pre-Pennsylvanian terrestrial life that built Seilacher (1963) has discussed the problems open, tubular shafts like these. of using trace fossils produced by invertebrates In contrast to the simple cylindrical tubes for the recognition of marine vs. nonmarine included in Category 1, Table 1, floodplain- depositional environments. He summarized dwelling insects also produce burrows with selected assemblages of nonmarine trace fossils terminal cells (Category 2, Table 1) which, if from the Lower to the Upper Trias- fossilized, would belong to the trace fossil ge- sic and concluded that the morphology of nus Macanopsis. It is hazardous, so far as in- many trace fossils is "independent of salinity," sects are concerned, to infer that there is any i.e. some of the same taxa could "occur in both ethological significance to the difference be- marine and freshwater environments." The tween Categories 1 and 2, Table 1; i.e. insect present authors would also extend this conclu- eggs are not always laid in cells and not all sion to terrestrial floodplains. For example, cells are used to deposit eggs. the surface of soft floodplain muds and sands Ichnologists also differ considerably with commonly contains large numbers of long, regard to the morphologic differences between shallow simple grooves (trails) with smoothly Planolites and Palaeophycus (Osgood, 1970, rounded transverse sections 1-20 mm across p. 375; Hantzschel, 1975, p. W95-W97; Frey BRETT C. RATCLZFFE AND J. A. FACERSTROM

and Chowns, 1972). The present authors have and insect produced lebensspuren with which used the criteria described by Alpert (1975) as we are familiar is unique to floodplains. the basis for our distinction (Categories 3 and 4. Among the floodplain lebensspuren we 4, Table 1). In our experience with Holocene have studied are forms which, if preserved in floodplain lebensspuren, both unbranched (po- the sedimentary record, would include the tential Pknolites) and branched (potential Pa- trace fossil genera Skolithos, Cylindricurn, laeophycus) horizontal, shallow, unpacked Sabellarifex, Macanopsis, Amphorichnus, tunnels commonly are found together. Both Planolites, Palaeophycus and Sinusites. Thus, Planolites and Palaeophycus are much better none of these genera in Pennsylvanian (when known from marine than from nonmarine en- the body fossil record of megascopic insects vironments. begins) or younger rocks may be indicative of Variation in burrow size for Holocene and marine vs. nonmarine environments. ancient shafts (Skolithos) and tunnels (Plan- olites; Palaeophycus) in sediments and rocks ACKNOWLEDGMENTS of both marine and nonmarine origin may be We would like to thank Lyle Klostermeyer the result of different species of burrow trace- and Gerald Konsler (both formerly graduate makers or of individuals at different ontoge- students at the University of Nebraska) for netic stages of the same species (Stanley and their interest and suggestions, and Martha J. Fagerstrom, 1974, p. 71, 80-81). Haack, Scientific Illustrator, University of Nebraska State Museum, for her illustrations CONCLUSIONS in Text-figures 1-4. C. Kent Chamberlain On the basis of our studies we conclude as shared his ideas on the origin of nonmarine follows: lebensspuren with the junior author on several 1. Holocene floodplain lebensspuren are occasions and also read the final draft of the very abundant locally and of great morpho- manuscript. Robert Frey and George Pember- logic diversity. However, relatively few of the ton (Department of Geology, University of forms have actually been reported in the fossil Georgia) reviewed the paper and offered valu- record. Insects may produce significant bio- able criticisms and suggestions. We are grate- turbation of both surficial and shallow intra- ful to Gail Littrell for typing the manuscript. stratal sediments. REFERENCES 2. The taxonomic diversity of burrowing spiders and insects inhabiting floodplains is Alpert, S. P. 1974. Systematic review of the genus Skolithos. J. Paleontol. 48:661-669. exceedingly high. We recognize burrowing -. 1975. Planolites and Skolithos from the Up- species included in 8 orders and 31 families per Precambrian-Lower Cambrian, White Inyo that produce lebensspuren capable of fossil- Mountains, California. J. Paleontol. 49508-52 1. ization. Insects and spiders do not ingest sed- Ahlbrandt, T. S., Sarah Andrews and D. T. iment nor do they line their trails and burrows Gwynne. 1978. Bioturbation in eolian deposits. with mucus; trace fossils produced by these J. Sed. Pet. 482339-848. Balduf, W. V. 1935. The Bionomics of Entomo- organisms should have rather poorly defined phagous Coleoptera. John S. Swift Co., St. walls that lack alteration "halos." Therefore, Louis, 220 p. where the trails and burrows of these organ- Bohart, G. E. 1952. Pollination by native insects, isms intersect, disruption of the earlier formed p. 107-121. In, Insects: The Yearbook of Agri- trace is sharply confined to the crossing point culture. U.S. Gov't. Print. Off., Wash., D.C., and does not extend into the sediment adjacent 780 p. -. 1964. Notes on the biology and larval mor- to the burrow. phology of strenua (Hymenoptera: 3. There is a remarkable convergence in Apoidea). Pan-Pac. Entomol. 40: 174-182. burrow morphology among taxonomically dis- Borror, D. J., D. M. DeLong and C. A. Triple- similar insects. Conversely, there may be con- horn. 1976. An Introduction to the Study of In- siderable dissimilarity among burrows made sects. Holt, Reinhart and Winston, Inc., N.Y., by the same species (or even individual) due 852 p. Cazier. M. A. and M. A. Mortenson. 1964. Studies to differences in ontogenetic stage, texture and on the bionomics of sphecoid wasps. I. Moniae- dampness of the substrate, weather, etc. (The cera asperata. Pan-Pac. Entomol. 40(2): 11 1-1 14. observations of Howard, 1976, lend additional - and -. 1965a. Studies on the bionomics support to this conclusion). None of the spider of sphecoid wasps. IV. Nitelopterus laticeps and FLOODPLAIN LEBE NSSPUREN 62 9

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