Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

FNZ 28 - Larvae of Curculionoidea (Insecta: Coleoptera) - Morphology

May, BM 1993. Larvae of Curculionoidea (Insecta: Coleoptera): a systematic overview. Fauna of New Zealand 28, 226 pages.
( ISSN 0111-5383 (print), ; no. 28. ISBN 0-478-04505-0 (print), ). Published 14 Jun 1993
ZooBank: http://zoobank.org/References/B6025BC7-2C53-4D46-B440-52C8F668DD5E

MORPHOLOGY

Diagnostic features

Curculionoid larvae are cylindrical grubs with the typical abdominal segments (Abd II-VII) unsclerotised and traversed on the dorsum by from two to four plicae or folds. Abd IX lacks urogomphi. A hypopharyngeal bracon is present, except in some leaf-miners. The maxilla has the galea and lacinia united to form a mala. Legs are vestigial or absent, rarely three-segmented with a claw (Nemonyx).

Larvae of Lucanidae, Trogidae, and Scarabaeidae are superficially similar to those of Curculionoidea apart from their well developed legs. The head capsule carries a hypopharyngeal bracon, their Abd segments are plicate and unsclerotised, and Abd IX lacks urogomphi. In Ptinidae, Anobiidae, Lyctidae, Bruchidae, and Chrysomeloidea (the closest out-group) the hypopharyngeal bracon is absent, and legs are present except in some Cerambycidae.

The antennae of all Curculionoidea except Belidae and Attelabidae are regarded as being one-segmented. The antennal segment itself is a membranous cushion which may be plane, convex, or cylindrical. It carries a supplementary cone as well as a variety of sensoria in the form of hairs, pores, or papillae, but since the cone is the most diagnostic structure it has become known as the 'antenna'. Some Belidae have a basal retractile membrane as in Cerambycidae, but this is not regarded as an additional segment.

Despite their uniform appearance curculionoid larvae present sharp character states, as detailed below, with the ancestral (plesiomorphic) state versus the derived (apomorphic) state. The modal numbers of setae for Curculionidae are used for comparison, and are listed in Table 3 (p. 50).

Head

  1. Free vs. retractile (dorsoepicranial seta 1 level with apex of frons, or forward of it) (Fig. 463).
  2. Extrusible vs. permanently retracted, held in position by strong musculature (as in Belidae, Fig. 197-199).
  3. Frontal sutures reaching mandible (Fig. 310, 336) vs. delimited in front by a frontoepicranial bridge (Fig. 387, 398)I
  4. Endocarinal line present (Fig. 2) vs. absent.
  5. Endocarinal line simple vs. divided (Fig. 209).
  6. Dorsoepicranial seta 3 on epicranial half vs. in suture or on frontal plate (Fig. 474, 506).
  7. Frontal setae exceeding 5 pairs vs. 5 pairs or fewer.
  8. Frontal setae comprising 5 pairs of similar length vs. fs1, 2, 3, 5 short or absent (fs4 is the most constant).
  9. Frontoclypeal suture distinct (Fig. 79) vs. effaced (Fig. 4).
  10. Clypeal setae comprising 3 or more pairs vs. not more than 2 pairs.
  11. Clypeolabral suture distinct vs. effaced.
  12. Antennae concealed from above (Fig. 35) vs. exposed.
  13. Antennae 2-segmented (Fig. 200) vs. 1-segmented (Fig. 26).
  14. Antennae circular in cross-section vs. oval.
  15. Antennae longer than wide vs. wider than long or subequal.
  16. Ocelli (stemmata) 3 or more (Fig. 23, 387) vs. not more than 2.

Mouthparts

  1. Mandibles with molar section undeveloped vs. developed (Fig. 29).
  2. Mandibles with 3 or more teeth on incisor section (Fig. 144) vs. 1 or 2 teeth.
  3. Labrum with 4 pairs of primary setae (Fig. 4) vs. not more than 3 pairs.
  4. Labrum transverse or quadrate vs. longitudinal (Fig. 854).
  5. Epipharyngeal lining with 4 or more pairs of anterolateral setae (Fig. 390, 400) vs. up to 3 pairs.
  6. Hypopharyngeal bracon clear vs. variously sclerotised (Fig. 443, 852).
  7. Hypopharyngeal bracon without a sclerome medially vs. with a complex medial sclerome (as in Anthribidae, Fig. 77).
  8. Maxillary palps 3-segmented (Fig. 204) vs. 2-segmented.
  9. Maxillary palps 2-segmented vs. 1-segmented (Fig. 351).
  10. Maxilla with palpiger (Fig. 204) vs. without palpiger.
  11. Maxillary mala bearing a lacinia (as in Anthribidae, Fig. 82) or an uncus (as in Nemonychidae, Fig. 17 ) vs. without lacinia or uncus.
  12. Articulatory lobes distinct (Fig. 7) vs. obsolete.
  13. Buccal setae simple vs. some setae branched (Fig. 1068b).
  14. Labial palps 2-segmented vs. 1-segmented (Fig. 360a).
  15. Labral tormae absent vs. present. (32) Tormae represented by lateral scleromes (Fig. 80) vs. paired paramesal rods.
  16. Tormae free vs. united at base (Fig. 615).

Thorax

  1. Pronotal shield simple vs. variously modified (Fig. 196).
  2. piracle on mesothorax or intersegmental vs. on prothorax.
  3. Legs or papillae present vs. absent.

Abdomen

  1. Spiracles 8 in number vs. fewer than 8.
  2. Spiracles with 2 airtubes (bicameral) vs. with 1 (unicameral) (Fig. 194).
  3. Spiracles with 1 airtube vs. none (Fig. 470).
  4. Spiracles located on pleurum vs. on dorsum (Fig. 788, 797).
  5. Dorsal transverse plicae (folds) 2 in number (Fig. 1) vs. 3 or 4 (Fig. 830, 949).
  6. Ventropleural lobes entire vs. subdivided (as in Rhynchophorinae, Fig. 1062).
  7. Abd VIII/IX simple vs. modified (Fig. 427).
  8. Abd X (anus) terminal vs. subdorsal (Fig. 611), subterminal (Fig. 558), or ventral (Fig. 548).
  9. Abd X simple vs. modified (e.g., pygopod, Fig. 23).

Alimentary canal

  1. Malpighian tubules 6 in number vs. 4.
  2. Rectal bracon a ligamentous ring vs. a sclerotised loop (Fig. 938).

Divergence between species shows in features such as pigmentation of head and setae and relative lengths of setae, and in the type and relative abundance of cuticular vestiture.

Evolution in larval morphology

The process of evolution as a result of cybernetics (Gk. kubernetes, steersman), or feedback from the environment, can be followed in various groups of organisms through sequences of morphological change when sufficient material is available. In weevils, evolution of the larvae has not necessarily proceeded at the same rate as in the adults. Because they usually live in sheltered situations, larvae are more likely to have remained constant, and can sometimes indicate a phylogenetic direction which has become uncertain in the adult, e.g., in the Australian Demyrsus Pascoe / Tranes Schoenherr complex (Molytini) (May, in press), and in the forest litter-inhabiting weevil Geochus Broun (see p. 80).

A classification system is a man-made artefact for presenting the multitudinous array of insects in an orderly fashion. The taxa are placed in a series of discrete compartments from where, because evolution is continuous, there is considerable overlap, especially at the family or subfamily level. Such transitional taxa can be recognised in weevil larvae, and will be discussed in the sections where they occur. Before assigning larvae to any particular grouping, however, one must remember that certain biological conditions can initiate synapomorphies in taxa which are not otherwise closely related. The following characters are the result of such parallel development, and should therefore not be given phylogenetic significance.

(1) Aquatic environments

Larvae which live in continuously wet surroundings have a modified respiratory system. The spiracles may be externally sclerotised and tipped with a scalpellum for plant piercing as in Cyrtobagous (Erirhininae), Steriphus (Rhytirhinini), and Donacia (Chrysomelidae). They may be arranged contiguously on the dorsum as in Cyrtobagous or laterally as in Neohydronomus (Erirhininae) and Steriphus. The number of functional spiracles may be reduced as in Neochetina (Erirhininae) and Donacia. The larvae of Notonesius (Phrynixini) living in wet peat have the pair of spiracles on the Abd VIII dorsum joined to form a telescopic siphon furnished with a float of aquifuge hairs to allow surface respiration. A similar strategy is employed by the maggots of Eristalis and other syrphid flies.

In watery habitats the respiratory system assumes major importance, and the hindgut is simplified so that Abd VIII, carrying the caudal spiracles, is often developed at the expense of Abd IX, carrying the anal musculature. Several of the foregoing genera and some Indian larvae inhabiting succulent plants (Gardner 1934a, 1938) have Abd IX reguced in size and Abd X, sometimes forming a pygopod, displaced well forward ventrally by the enlargement of Abd VIII. Free-drifting larvae such as those of Cyrtobagous display various setal modifications. The long, slender pleural setae, especially on the terminal segments, probably function as balancers whereas the stout, hooked ventral setae are used as a holdfast while feeding.

(2) Ectophytic feeding

Larvae which feed externally are adapted for adhesion to plane surfaces by means of ventral ambulatory ampullae, as in Listroderes (Rhytirhinini) and Hypera (Hyperini); the anal lobes are developed as pygopods as in Listroderes and Rhinorhynchus (Nemonychidae); and setation may be reduced or obsolete, and coloration is cryptic, as in Gonipterini and the previous genera.

(3) Xylophagy

Larvae which feed on the hard, resistant parts of dead wood show characteristic modifications to the mouthparts: labrum longer than wide, narrowed in front and heavily sclerotised (pigmented); epipharyngeal lining with als arranged longitudinally; mandibles with cutting edge raised medially and either ridged or knobbled to form a grinding surface; and the hypopharyngeal bracon solidly pigmented for added strength. Examples of larvae with mouthparts adapted for xylophagy may be found in Psepholax and its allies (Cryptorhynchini), Phrynixus and its allies (Molytini), and Ancistropterus (Eugnomini), all in Curculioninae, in Mesites (Cossoninae), and in Pachycotes (Scolytinae).

(4) Leaf-mining

Larvae which feed in the constricted space between leaf surfaces are subject to considerable modification of the head capsule and labrum, the degree of which varies between genera in a well defined regressive cline. The head is retracted to bring the mandibles into a forward position, and depressed to fit between the upper and lower leaf cuticle. The posterior emargination of the head is increased dorsally so that the coronal suture is progressively eliminated. The endocarina becomes stronger, and extended to the full length of the frons and then posteriorly as a rod between the open epicranial halves.

Cephalic setae become short and eventually obsolete in the following sequence: des4, fsl; des2, fs2,3; desl, fs5, les2, ves1,2; des3, fs4. Setae des5 and lesl remain constant. Labral setae become short and acute, reducing to one pair only. The epipharyngeal setae, however, in a protected position, are well developed and in more or less modal numbers.

Of the seventeen endemic curculionines known to be leafminers, six have been examined in detail. Phorostichus linearis (Broun) in Astelia and Collospermum shows the least amount of modification, in comparison with Peristoreus Kirsch, followed by Peristoreus (pending revision) flavitarsis (Broun) in Podocarpus totara, Hypotagea concolor (Broun) in Nothofagus, Notinus aucklandicus Kuschel in Coprosma and Nertera, Neomycta rubida Broun in Metrosideros, and finally Geochus tibialis Broun in dead leaves of Weinmannia with only two cephalic setae remaining and with the endocarina extended posteriorly. The likeness of Geochus, in these respects, to the European Rhamphus Clairville and Rhynchaenus Clairville suggests that a similar cline may exist among leafminers in the Northern Hemisphere.

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