Phthiraptera

Introduction

Lice belong to the order Phthiraptera, and are the only truly parasitic group amongst the exopterygote insects. As permanent ectoparasites of most birds and mammals they exhibit a remarkable level of host specificity which is unparalleled in most other metazoan parasites. Most individuals will spend their entire life cycle on a single host, with transmission largely occurring opportunistically when hosts are in close contact with each other, such as during breeding (but see also phoresy). This unique lifestyle has led to numerous adaptations according to their precise ecological niche on the host, consequently lice are considerably diverse in terms of their size and general body form. Specializations in the diet of lice underpin their major taxonomic divisions and they can be broadly separated into those that feed on skin debris, feathers and fur, and those that have specialized in blood feeding.

To date there are more than 3000 known species of lice and yet many more remain undescribed. With the possible exception of those species that impinge on the activity of humans and their livestock, the true biology of this cryptic group of insects remains obscure, but their remarkable host specificity has attracted much recent attention by evolutionary biologists interested in the ecology and cospeciation of lice and their hosts.

Characteristics

• Wingless (apterous) ectoparasites
• Three nymphal instars
• Hemimetabolous (having a simple metamorphosis, i.e. no pupa)
• Mouthparts - mandibles for chewing lice, stylets for sucking lice
• Tarsi 1 or 2 segmented (1 in Anoplurans)
• Dorsoventally flattened head
• Development of operculum in egg
• Reduced labial palps
• Reduced compound eyes and no ocelli
• Antennae are 3 to 5 segmented and capitate
  • Recessed into the head in the Amblycera
  • Filiform in the Ischnocera (May be modified as clasping organs in the male)
  • Typically short in Anoplura

Classification

The order Phthiraptera has been traditionally divided into two groups according to their different feeding habits: the chewing lice or "Mallophaga", and the Anoplura, colloquially known as the sucking lice. It is commonly assumed that the order is derived from a primitive Pscopteran-like ancestor which became parasitic first on birds.

Chewing lice with their large head and mandibles comprise the largest group with some 2900 species. These are separable into three distinct superfamilies - the Amblycera, Ischnocera and Rhyncophthirina. Amblycerans are found on a diverse range of mammals and birds, and have retained some of the more primitive characteristics of the order, with a lesser degree of specialization for particular habitats. This is reflected in their classification, and they divided into around sixty homogenous genera, whilst the Ischnocera are contained by over a hundred and fifty genera, of which around three quarters are confined to birds. Amblycera chew away at younger feathers and soft areas of the skin, causing localized bleeding from which some can drink. They usually roam freely about the surface of their host and seldom attach firmly to fur or feathers, unlike the avian Ischnocera which are more site specific, and will attach securely to the feathers or fur to escape the preening activity of their host. Ischnocera confine themselves to feeding on the downy part of feathers and softer fur. The Rhyncophthirina comprises a single genus which parasitise elephants and wart-hogs. They are a small louse carrying mandibles at the end of a long proboscis-like snout. This allows them to drill through their hosts thick skin.

Anoplura are a much smaller group comprised of some 500 species. These are restricted to mammals, and like the Rhyncophthirina, feed using maxillae positioned at the end of a snout-like protrusion to pierce the skin. Feeding solely on blood they remain at the feeding site causing localized skin irritations to their host. Because of this they are the vectors to a number of blood borne diseases. This group includes the human louse Pediculus humanus, consequently they are probably the most well studied louse group.

Discussion of Phylogenetic Relationships

The phylogenetic relationships and classification of the four main groups of lice have been matters of contention for some time. A cladistic analysis of the morphological data conducted by Lyal (1985) detailed objections to the traditional classification of lice into "Mallophaga" and Anoplura. His study supported the monophyly of the Phthiraptera, with Anoplura and Rhyncophthirina forming a monophletic group, sister group to the Ischnocera; the Amblycera are the sister group of this assemblage. This working hypothesis seems to have received general support amongst most leading lice systematists. Nevertheless, the familial relationships amongst most major groups of lice are poorly understood, and resolution of these relationships awaits an analysis of all available morphological data, as well as the gathering of molecular information. These relationships will not be easy to uncover as many groups have had a long and complex independent history, frequently involving instances of parallel and convergent evolution.

Other Names for Phthiraptera

• Vernacular Names: Parasitic lice

References

Clay, T. 1949. Some problems in the evolution of a group of ectoparasites. Evolution 3: 279-299.

Clay, T. 1951. An introduction to a classification of the avian Ischnocera (Mallophaga): Part I. Transactions of the Royal Entomological Society of London 102,2:171-195.

Clay, T. 1970. The Amblycera (Phthiraptera: Insecta). Bulletin of the British Museum of Natural History Entomology 25: 75-98.

Demastes, J. W. and M. S. Hafner 1993. Cospeciation of pocket gophers (Geomys) and their chewing lice (Geomydoecus). Journal of Mammalogy 74(3): 521-530.

Durden, L. A. and G. G. Musser 1994. The sucking lice (Insecta, Anoplura) of the world: A taxonomic checklist with records of mammalian hosts and geographic distributions. Bulletin of the British Museum of Natural History 218: 90 pp.

Hafner, M. S. and R. D. M. Page 1995. Molecular phylogenies and host-parasite cospeciation: gophers and lice as a model system. Philosophical Transactions of the Royal Society of London. Series B. 349: 77-83.

Hopkins, G. H. E. and T. Clay 1952. A checklist of the genera and species of mallophaga. London, British Museum of Natural History.

Lakshminarayana, K. V. 1986. Data book for the study of chewing lice (Phthiraptera : Insecta). Records of the Zoological Survey of India: Miscellaneous Publication, Occasional Paper 81: 1-63.

Lyal, C. H. C. 1985. A cladistic analysis and classification of trichodectid mammal lice (Phthiraptera: Ischnocera). Bulletin of the British Museum (Natural History) 51(3): 187-346.

Page, D. M., R. D. Price, et al. 1995. Phylogeny of Geomydoecus and Thomomydoecus pocket gopher lice (Phthiraptera: Trichodectidae) infered from cladistic analysis of adult and first instar morphology. Systematic Entomology 20: 129-143.

Page, R. D. M. 1994. Parallel phylogenies: Reconstructing the history of host-parasite assemblages. Cladistics 10: 155-173.

Pilgrim, R. L. C. and R. L. Palma 1982. A list of the chewing lice (Insecta: Mallophaga) from birds in New Zealand. National Museum of New Zealand Miscellaneous Series 6: 1-32.

Rothschild, M. and T. Clay (1952). Fleas, flukes & Cuckoos: A study of bird ectoparasites. Colins (New Naturalist Series).London.

R?zsa, L. 1993. Speciation patterns of ectoparasites and "straggling" lice. International Journal for Parasitiology 23(7): 859-864.

Information on the Internet

• SEM Picture Library - Lice Index
• Louse Taxonomy and Phylogeny from Glasgow University
• Lice Pictures from Iowa State University Entomology Image Gallery
• California Academy of Sciences Phthiraptera Type Collection