Evolution of the Insects

CY501-C11[407-467].qxd 3/2/05 12:56 PM Page 407 quark11 Quark11:Desktop Folder:CY501-Grimaldi:Quark_files: But, for the point of wisdom, I would choose to Know the mind that stirs Between the wings of Bees and building wasps. –George Eliot, The Spanish Gypsy 11HHymenoptera:ymenoptera: Ants, Bees, and Ants,Other Wasps Bees, and The order Hymenoptera comprises one of the four “hyperdi- various times between the Late Permian and Early Triassic. verse” insectO lineages;ther the others – Diptera, Lepidoptera, Wasps and, Thus, unlike some of the basal holometabolan orders, the of course, Coleoptera – are also holometabolous. Among Hymenoptera have a relatively recent origin, first appearing holometabolans, Hymenoptera is perhaps the most difficult in the Late Triassic. Since the Triassic, the Hymenoptera have to place in a phylogenetic framework, excepting the enig- truly come into their own, having radiated extensively in the matic twisted-wings, order Strepsiptera. Hymenoptera are Jurassic, again in the Cretaceous, and again (within certain morphologically isolated among orders of Holometabola, family-level lineages) during the Tertiary. The hymenopteran consisting of a complex mixture of primitive traits and bauplan, in both structure and function, has been tremen- numerous autapomorphies, leaving little evidence to which dously successful. group they are most closely related. Present evidence indi- While the beetles today boast the largest number of cates that the Holometabola can be organized into two major species among all orders, Hymenoptera may eventually rival lineages: the Coleoptera ϩ Neuropterida and the Panorpida. or even surpass the diversity of coleopterans (Kristensen, It is to the Panorpida that the Hymenoptera appear to be 1999a; Grissell, 1999). Already it is known that in well-sampled related, owing to the reduction into a single claw on the larval temperate regions, Hymenoptera outnumber Coleoptera leg, the presence of labial silk glands in the larva, and a scle- (e.g., Gaston, 1991), and the order rivals the beetles in a few rotized sitophore plate in the cibarium of the adult mouth- well-studied tropical ecosystems (e.g., Noyes, 1989; Stork, parts (Königsmann, 1976; Kristensen, 1981, 1991, 1995, 1999a), 1991; Hanson and Gauld, 1995). Today there are approxi- as well as significant reduction of the prothorax. Molecular mately 125,000 named species of Hymenoptera, but even the studies have also supported this position (e.g., Whiting et al., most conservative estimates on the total diversity of the 1997; Wheeler et al., 2001). The sister-group relationship order approximate figures between 600,000 and 1,200,0000 between the Hymenoptera and the panorpid orders is species (Gaston, 1991; Grissell, 1999; Austin and Dowton, intriguing and might, on the surface, suggest that the order 2000b). The sobering aspect of these estimates is that 60–88% should stem well into the Permian because numerous Paleo- of the order remains undescribed, perhaps 96% if Stork’s rad- zoic mecopteroids (stem-group Antliophora and Panorpida) ical estimate of 2.5 million species is adopted (Stork, 1996). are known from this period. This vast diversity is largely hidden among the relatively The earliest definitive Hymenoptera, which are easily rec- poorly explored microhymenopterans (e.g., Proctotrupoidea, ognizable by the distinctive wing venation, are from the Chalcidoidea, Platygastroidea) and where vast new faunas of Triassic of Australia, Central Asia, and Africa (Riek, 1955; species are continually discovered. However, even among Rasnitsyn, 1964, 1969; Schlüter, 2000). Are the Hymenoptera the more physically “robust” lineages, like Ichneumonoidea, more ancient than presumed, owing to the antiquity of their the number of species is rising rapidly as more comprehen- sister group? They likely are not. As we shall discuss later, sive sampling and monographs are undertaken of megadi- numerous fossilized “mecopteroid” wings from the Permian verse regions, such as Malaysia, Indonesia, Ecuador, Peru, exhibit only primitive traits for the Hymenoptera ϩ Panorp- and Zaire. Major treatments covering both the systematics ida complex, and defining features of Panorpida cannot and biology of the Hymenoptera include Evans and West- be determined for many of these taxa. It is, therefore, pos- Eberhard (1970), Iwata (1972, 1976), Spradberry (1973b), sible that some of these Paleozoic groups are in fact stem- Krombein et al. (1979), Gauld and Bolton (1988), Schedl group lineages to the Hymenoptera ϩ Panorpida, with the (1991), Goulet and Huber (1993), LaSalle and Gauld (1993), Hymenoptera and panorpoid orders as we know them today Godfray (1994), Hanson and Gauld (1995), Quicke (1997), having arisen from a paraphyletic cloud of Paleozoic taxa at Austin and Dowton (2000a), and O’Neill (2001). Although 407 CY501-C11[407-467].qxd 3/2/05 12:57 PM Page 408 quark11 Quark11:Desktop Folder:CY501-Grimaldi:Quark_files: 408 EVOLUTION OF THE INSECTS of the union of two gametes. Males, however, are produced from unfertilized eggs and as such have only a single com- pliment of the genome (i.e., are haploid). Diploid males do at times occur, particularly among the social aculeates, but typically they do not survive to maturity, but instead die as larvae. Hymenoptera are well known as parasitoids (Figure 11.2), a mode of life in which they have succeeded and far sur- passed all other insect parasites. As such, the group is of para- mount importance in natural ecosystems, but it has also made them a focus for biological control agents in integrated pest management (e.g., Waage and Greathead, 1986). Para- sitoids, unlike parasites, develop from nutrients extracted from a single host organism either externally (ectopara- sitoids) or internally (endoparasitoids), and they kill the host 11.1. Hamuli on the hind wing of a wasp, which is a defining feature as a direct or indirect result (a parasite, while inflicting mini- of the Hymenoptera. These microscopic hooks link to the posterior mal to severe ill effects, does not kill its host). The host, how- margin of the forewing and couple the wings in flight. Hymenoptera ever, remains alive for the larger part of the parasitoid’s effectively fly like flies, with functionally one pair of wings. Scanning electron micrograph. period of feeding. This mode of life is remarkably efficient, and parasitoid wasps are so diverse that they exploit most groups of terrestrial arthropods. Some parasitoid wasps are certainly dated, the book by Malyshev (1966: translated 1968) hyperparasitoids, in which one parasitoid attacks another is a remarkable source for information on the biology of hymenopterans. As mentioned before, the Hymenoptera possess numerous primitive traits combined with several unique features. Notable primitive features in the groundplan of Hymenoptera include the mandibulate mouthparts and the generalized ovipositor of the “lepismatoid” type (i.e., the typical arrange- ment for Dicondylia with a gonangulum), although the first gonocoxite in the order has been entirely lost and is wholly replaced by the gonangulum (and comments about a first valvifer or gonocoxa in the order are erroneous and actually refer to the gonangulum) (Figure 4.8). Hymenoptera are defined by the following specialized features: relatively small hind wings that are linked to the forewings by a series of minute hooks (hamuli) on the leading edge and that grasp the posterior edge of the forewing (Figure 11.1); the anal veins of the forewing not reaching the posterior wing margin; a modification of the protibial spur into an “antenna cleaner”; articulation of the profurcal apophyses to the propleura; the presence, at least among basal hymenopterans, of cenchri (paired oval structures on the metascutum, which can engage the forewing undersides in symphytans); and the presence of volsellae in the male genitalia (other morphological traits are outlined by Vilhelmsen, 2001; Schulmeister, 2003b). Perhaps one of the more interesting, derived traits of the Hymenoptera, and one that plays an important role in kin selection throughout the social lineages, is a haplodiploid sex determination system, though this also occurs sporadically throughout other groups of 11.2. Parasitoid wasps emerging from their beetle larva host. The order Hymenoptera contains the greatest diversity and numbers of insects. In all members of the order, females have two sets of insect parasitoids, which are largely responsible for regulating popula- chromosomes (i.e., are diploid), being the usual product tions of pest insects and so are used extensively in biocontrol. CY501-C11[407-467].qxd 3/2/05 12:57 PM Page 409 quark11 Quark11:Desktop Folder:CY501-Grimaldi:Quark_files: HYMENOPTERA: ANTS, BEES, AND OTHER WASPS 409 11.3. Phylogenetic relationships of the sawflies and wood wasps (symphytan Hymenoptera). Numbers refer to some significant characters (see Table 11.1). Based on Vilhelmsen (2001) and Schulmeister (2003b), with fos- sils added. Crosshatching indicates putative ranges which require more careful authentification. the phylogeny and classification of Hymenoptera is presently TABLE 11.1. Significant Characters in the Phylogeny the subject of intense study, and revisions to this long dated of Basal Hymenopteraa system are expected in due course. Rather than adopt a novel 1. Hamuli; protibial spur with velum; haplodiploid subordinal classification in advance of such studies, we shall 2. Forewing Rs not furcate apically, Sc fused to R; Sc absent refer to these imperfect subdivisions but shall

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