Invertebrate Anatomy OnLine
Scarabaeid Beetle Larvae ©
Copyright 2004 by
is an exercise from Invertebrate
Anatomy OnLine , an
Internet laboratory manual for courses in Invertebrate Zoology. Additional
exercises can be accessed by clicking on the links to the left. A
glossary and chapters on supplies and laboratory techniques are also available. Terminology
and phylogeny used in these exercises correspond to usage in the Invertebrate
Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated
figure callouts refer to figures in the textbook. Callouts
that are not hyphenated refer to figures embedded in the exercise. The glossary
includes terms from this textbook as well as the laboratory exercises.
Mandibulata sP, Tracheata,
Hexapoda SC, Insecta C,
Dicondylia, Pterygota, Metapterygota, Neoptera, Eumetabola, Holometabola,
Coleoptera O, Polyphaga sO,
Scarabaeidae F, Rutelinae sF (Fig
16-15, 20-14, 20-15, 21-23)
by far the largest and most diverse animal taxon, includes chelicerates,
insects, myriapods, and crustaceans as well as many extinct taxa. The
body is segmented and primitively bears a pair of jointed appendages on each
epidermis secretes a complex cuticular exoskeleton which must be molted to
permit increase in size. Extant
arthropods exhibit regional specialization in the structure and function of
segments and appendages. The body is typically divided into a head and trunk, of
which the trunk is often itself divided into thorax and abdomen.
gut consists of foregut, midgut, and hindgut and extends the length of the body
from anterior mouth to posterior anus. Foregut
and hindgut are epidermal invaginations, being derived from the embryonic
stomodeum and proctodeum respectively, and are lined by cuticle, as are all
epidermal surfaces. The
midgut is endodermal and is responsible for most enzyme secretion, hydrolysis,
coelom is reduced to small spaces associated with the gonads and kidney. The
functional body cavity is a spacious hemocoel divided by a horizontal diaphragm
into a dorsal pericardial sinus and a much larger perivisceral sinus. Sometimes
there is a small ventral perineural sinus surrounding the ventral nerve cord.
hemal system includes a dorsal, contractile, tubular, ostiate heart that pumps
blood to and from the hemocoel. Excretory
organs vary with taxon and include Malpighian tubules, saccate nephridia, and
organs also vary with taxon and include many types of gills, book lungs, and
nervous system consists of a dorsal, anterior brain of two or three pairs of
ganglia, circumenteric connectives, and a paired ventral nerve cord with
segmental ganglia and segmental peripheral nerves. Various
degrees of condensation and cephalization are found in different taxa.
is derived with centrolecithal eggs and superficial cleavage. There
is frequently a larva although development is direct in many. Juveniles pass
through a series of instars separated by molts until reaching the adult size and
reproductive condition. At
this time molting and growth may cease or continue, depending on taxon.
includes arthropods in which the third head segment bears a pair of mandibles. As
currently conceived this taxon includes myriapods, hexapods, and crustaceans. Appendages
may be uni- or biramous and habitats include marine, freshwater, terrestrial,
and hexapods share tracheae and a single pair of antennae and are sister taxa in
which have gills and lack tracheae, are excluded and form the sister group.
body is divided into three tagmata; head, thorax, and abdomen. Appendages
are uniramous and a single pair of antennae is present. Three
pairs of legs and two pairs of wings are found on the thorax of most adults. Hexapod
legs are uniramous although there is increasing evidence that they evolved from
multiramous appendages of their ancestors. Gas
exchange is accomplished by trachea. Excretory
organs are Malpighian tubules and the end product of nitrogen metabolism is uric
is relatively little cephalization of the nervous system. Insects are gonochoric
with copulation and internal fertilization.
hexapods are insects. A
few hexapod taxa (orders) lack wings and have primitive mouthparts recessed into
the head and belong to Entognatha, the sister taxon of Insecta. Insects
have ectognath mouthparts and the adults (imagoes) of most taxa have wings.
winged insects. These insects are derived from a winged common ancestor. Adults
of most taxa have wings although they have been lost in some.
have no ocelli and there are six or fewer Malpighian tubules.
final larval instar pupates and undergoes a radical metamorphosis in which it is
converted to an imago, or adult. The
imago is sexually mature and in most taxa has wings whereas larvae are immature
and wingless. During
metamorphosis many or most larval tissues are dismantled and adult structures
built anew. Wings,
for example, are manufactured from clusters of undifferentiated cells known as
imaginal discs but not from preformed wingpads as in pauro- and hemimetabolous
is the largest taxon (of equivalent rank) of organisms with about 300,000
described Recent species. The
body of most beetles is heavily sclerotized. The
forewings are sclerotized elytra, or wing covers. Beetle larvae are known as
grubs and do not resemble adults.
larvae of many species of scarab beetles inhabit turf grasses and are
collectively known as white
grubs. They are similar to each other, white or grey, with a darker
posterior abdomen, and brown head capsule. They curl into a characteristic
C-shape (Fig 1, 21-14).
Figure 1. A Cyclocephala (masked
chafer) grub viewed from the left side. Beetle71L.gif
common species of white grubs in the eastern United States are Japanese beetle (Popillia
japonica), northern masked chafer (Cyclocephala borealis),
southern masked chafer (Cyclocephala lurida), green June beetle (Cotinus
nitida), May-June Beetle (Phyllophaga fusca), European chafer (Rhizotrogus
majalis), Asiatic garden beetle (Maladera castanea), Oriental
beetle (Exomala orientalis), false Japanese beetle (Strigoderma
arbicola), and black turfgrass ataenius (Ataenius spretulus). The
subterranean larvae feed on the roots of turf grasses and other plants whereas
the adults consume the foliage of a variety of trees and shrubs. These grubs are
distinguished from each other by the shape of the anus and pattern of spines on
the venter of the tenth (last) abdominal segment.
descriptions and drawings for this exercise are based on observations of masked
chafer larvae (Cyclocephala) but apply, in almost all details, to any
larvae of this group. If you are interested in species identification, refer to
the Web links in the bibliography. Green
June beetle grubs have the interesting habit of crawling upside down, i.e. on
their backs, over the ground surface and will do so if you place one on the
ground or in a culture dish in the lab.
Figure 2. En
face view of the head capsule of
a Cyclocephala grub.
can be collected from suburban lawns and gardens by digging in the sod and
underlying soil. The
following collecting technique minimizes damage to the sod. Cut
through the sod on three sides of a 30-cm square and roll the turf away from the
soil along the fourth, uncut, edge of the square. Remove
the underlying, now exposed, 5 cm of soil and pass it through a 5-mm sieve to
retain any grubs present. Replace
the soil, move the sod back into position, and water it. In
dry weather the sod should be watered a day in advance of collecting to attract
the grubs from lower, moister levels of the soil.
collected grubs can be studied alive after anesthetization with chloroform or
they can be preserved with 40% isopropanol or 80% ethanol. The
exercise is confined to external anatomy and, although written specifically for Cyclocephala,
can be used for the larvae of any scarab beetle.
beetle grubs are white, with a brown head capsule and a brownish posterior end. They
are about 25-40 mm in body length and 4-6 mm in diameter. The
wormlike body is curved in a "C" shape (Fig 1). The
body is divided into the usual three insect tagmata; head, thorax,
Figure 3. Left lateral view of the head of Cyclocephala.
head consists of a dark, well-developed, sclerotized head
capsule equipped with sense
organs and mouthparts (Fig 2, 3). The vertex is
the dorsal region of the capsule and it grades imperceptibly into the frons, the
front of the capsule. The clypeus articulates
with the ventral edge of the frons. The sides of the capsule are the genae,
or cheeks (Fig 3).
the appendages of the head observing them first in anterior view and then in
posterior view as required. The antenna arises
from the ventral edge of the gena. No
compound eyes are present but a tiny black ocellus is
barely visible through the cuticle at the base of the antenna.
the mouthparts, only the labrum and mandibles are visible in anterior view. The
oval, but asymmetrical, labrum articulates
along a straight suture with the ventral edge of the clypeus (Fig 2, 3). The
posterior surface of the labrum bears the epipharynx,
prominent, median, posteriorly directed tooth that can be seen only in posterior
view (Fig 4). The
labrum is not bilobed.
massive mandibles articulate
laterally with the ventral edges of the genae (Fig 2, 3). The heavily
sclerotized mandibles each have a distal incisor for
shearing and a massive proximal molar,
or molars, for grinding (Fig 4). Lift the labrum to reveal the full extent of
the heavily sclerotized mandibles. The molars are best seen in posterior view
after moving (or removing) the labium and maxillae.
mandible articulates with the gena via two contact points, or condyles. The dorsal and ventral
condyles can be seen in
lateral view at the border of the clypeus (Fig 3). A
dicondylic articulation such as this limits movement to a single plane. The
mandibles can move only in the transverse plane and thus oppose each other
across the midline, incisor opposite incisor and molar opposite molar.
Dicondylic articulations are typical of insect mandibles and several of the
joints of insect legs. The human knee and elbow joints are also examples of
4. Posterior view of the mandibles and labrum. Maxilla
and labium removed for clarity. Beetle74L.gif
the posterior face of the mandible, and visible only with the maxillae and
labium out of the way or removed, is an oval patch, the stridulating
plate, with fine striae (Fig 4). Higher
magnification, about 40X or more, will be required to resolve the striae. The
stridulating plate of the mandible is half of a stridulating device used by the
grub to produce sound. The
device consists of a file and scraper similar to those on the wings of male
crickets (Acheta). The file is
the stridulating plate of the mandible and the scraper is a row of bumps on the
maxilla (Fig 6). The
scraper lies against the file when the mandible and maxilla are in their natural
positions. To stridulate, the grub rubs the scraper over the file. Also
visible in posterior view of each mandible is an accessory condyle extending
from the base of the mandible.
maxilla is best viewed from the posterior side of the head, using fine forceps
and a microneedle to manipulate it. The
two maxillae are lateral to the labium so the three are simultaneously visible
in posterior view (Fig 5). Each maxilla consists
of a basal cardo articulated
with the head capsule proximally and the stipes distally.
the stipes arise a lateral maxillary
palp and a fused galea and lacinia (Fig
5, 6). The presence of both galea and lacinia is apparent only in anterior view
(Fig 6). In
posterior view the two appear to be a single entity. The galea is lateral and
the lacinia medial. The combined galea-lacinia bristles with stout spines and
5. Posterior view of the maxillae and labium of Cyclocephala.
Figure 6. Anterior view of the labium and maxillae of Cyclocephala.
anterior edge of the stipes bears a longitudinal ridge of sclerotized tubercles
known as stridulating teeth (Fig
6). The stridulating teeth form the scraper that
is the complement of the mandibular file of the stridulating organ (Fig
at an intact labium in
posterior view. It
is median, between the two maxillae (Fig 5). It
consists of a proximal submentum and
a distal prementum with
a mentum in between. Two tiny biarticulate labial
palps arise distally from
the mentum (Fig 3).
or tongue, in most insects is a soft, unsclerotized fold of the ventral wall of
the head posterior to the mouth. In
white grubs it is fused with the prementum of the labium and is heavily and
asymmetrically sclerotized (Fig 6). Its
black cuticular parts are clearly visible on the anterior base of the labium.
Figure 7 Left lateral view of thorax. Setae
omitted for clarity. Beetle77L.gif
in the larvae or most holometabolous insects the posterior tagmata, thorax and
abdomen, are weakly sclerotized making it difficult to distinguish segments. The
body is soft and worm-like.
always, the thorax consists of 3 segments which are the prothorax, mesothorax,
and metathorax (Fig
1, 7). Each thoracic segment bears a pair of walking legs known, in order, as
the forelegs, midlegs,
and hindlegs (Fig
are similar and each consists of coxa, trochanter, femur, tibia, tarsus, and
single tarsal claw. A pair of spiracles is
present laterally on the prothorax (Fig 1, 7).
abdomen consists of 10 segments but
they are soft and mostly without distinct sclerites. Abdominal segments 1-8 each
have a pair of spiracles (Fig
1) but spiracles are not present on segments 9 and 10. In Cyclocephala,
but not Phyllophaga, segment
10 has a slender sclerotized tergite arching
dorsally over the segment. The anus is
a horizontal, transverse slit extending across the posterior end of segment 10.
The region of segment 10 ventral to the anal slit is known as the raster and
it bears a species-specific pattern of short spines (Fig 1, 8).
Figure 8. Ventral view of the posterior abdomen
showing the random setal pattern characteristic of the raster of Cyclocephala. Setae
have been omitted from the dorsum of segment 10. beetle78L.gif
identification of white grubs is accomplished by county extension agents, turf
grass managers, and homeowners using the pattern of short setae or spines on the raster,
or ventral surface of abdominal segment 10 (Fig 8). The
pattern is discernable in the field using a 10X lens. In the two species of
chafers (Cyclocephala) the pattern of spines is random (Fig 8). Some of
the spines of Japanese beetles (Popillia), on the other hand, form a
distinct "V" immediately anterior to the anal slit. May-June beetles (Phyllophaga)
have two parallel, or slightly curved, closely spaced rows of spines in
conjunction with a doubly arched anal slit (Fig 9). Green
June beetles (Cotinus) have several long parallel rows of spines. Refer
to the Internet sites below for illustrations of these, and other, raster
Figure 9. Ventral
view of the posterior abdomen showing the random setal pattern characteristic of
the raster of a May-June beetle, Phyllophaga.
Anon. undated. University
of Kentucky Agripedia, Entomology 320 Horticultural Entomology.
Böving AG, Craighead
An illustrated synopsis of the principal larval forms of the order Coleoptera.
Entomologica Americana 11(ns)(1):1-244..
Böving AG, Craighead
An illustrated synopsis of the principal larval forms of the order Coleoptera.
Entomologica Americana 11(ns)(4):257-352.
Borrer DJ, Triplehorn
CA, Johnson NF. 1989. An
introduction to the study of insects, 6 th ed. Saunders,
Hudson WH, Sparks B. undated.
White grub pests on turfgrass. http://pubs.caes.uga.edu/caespubs/pubcd/1428-w.html
Ruppert EE, Fox RS,
Barnes RB. 2004.
Invertebrate Zoology, A functional evolutionary approach, 7 th ed.
Brooks Cole Thomson, Belmont CA. 963 pp.
Russell H .
2001. Identifying white grubs in Michigan. www.pestid.msu.edu/profiles/whitegrubidentification.html With
good photos and explanations of raster patterns for identifying white grubs.
Sheltar DJ .
2000. Identification of white grubs in turfgrass.
living or preserved white grub (scarab beetle larva)
dissecting set with fine forceps and microneedles
small dissecting pan (anchovy tin)