INSECTIVORA, an order of non-volant placental mammals of small size, with a dentition adapted to an insect-diet. In nearly all cases these creatures are nocturnal, and the majority are terrestrial, many burrowing in the ground, although a few are arboreal and others aquatic. They have plantigrade or partially plantigrade feet, that is to say, they apply the whole or the greater portion of the soles to the ground when walking; and there are generally five toes, each terminating in a claw, and the first never being opposable to the others in either the fore or hind limb. A full series of differentiated teeth, including temporary or deciduous milk-molars, is developed, and the cheek-teeth have distinct roots and are crowned with sharp cusps, which in some instances are three in number and arranged in a triangle. Very frequently the number of the teeth is the typical forty-four, arranged as i. 3⁄3, c. 1⁄1, p. 4⁄4, m. 3⁄3, but occasionally there is a fourth pair of molars, while the incisors may be reduced to two pairs above and one below, and the canine is frequently like an incisor or a premolar. The skull is of a primitive type, often with vacuities on the palate, as in marsupials, with a small brain-chamber, and the tympanic bone generally ring-like instead of forming a bladder-shaped bulla; except in the African Potamogale, clavicles, or collar-bones, are always present; the humerus generally has a perforation on the inner side of its lower extremity; and a centrale bone is usually present in the carpus. In the brain the smooth hemispheres are so short as to leave the cerebellum and sometimes even the corpora quadrigemina exposed. The uterus is two-horned; the placenta, so far as known, is deciduate and discoidal; the testes are abdominal or inguinal; and the teats usually numerous. The body in several instances is covered with sharp spines in place of hair.
The great majority of the Insectivora are nocturnal in their habits, and their whole structure indicates an extremely low grade of organisation, fully as low as that of marsupials. It is noteworthy that the dentition in several of the groups approximates to that of the extinct mammals of the Jurassic epoch (see [Marsupialia]), and exhibits more or less distinctly the primitive tritubercular type. Although the past history of the group is very imperfectly known, it seems probable that the Insectivora are nearly related to the original primitive mammalian stock. Indeed, it has been stated that were it not for the apparently advanced type of placenta, they might easily be regarded as the little modified descendants of the ancestors of most other mammals. Probably they are in some way related to the creodont carnivores (see [Creodonta]), but if, as has been suggested, the latter are akin to the primitive ungulates, the connexion would seem to be less close than has been sometimes supposed.
Representatives of this order are found throughout the temperate and tropical parts of both hemispheres, with the exception of South America (where only a few shrews have effected an entrance from the north) and Australia, and exhibit much variety both in organization and in habit. The greater number are cursorial, but some (Talpa, Chrysochloris, Oryzorictes) are burrowing, others (Limnogale, Potamogale, Nectogale, Myogale) aquatic, and some (Tupaiidae) arboreal. To the great majority the term insectivorous is applicable, although Potamogale is said to feed on fish, and the moles live chiefly on worms. Notwithstanding the nature of their food, much variety prevails in the form and number of the teeth, and while in many cases the division into incisors, canines, premolars and molars may be readily traced, in others, forming the great majority of the species, such as the shrews, this is difficult.
In most cases the brain-cavity is of small relative capacity, and in no instance is the brain-case elevated to any considerable extent above the face-line. The facial part of the skull is generally much produced, and the premaxillary and nasal bones well developed; but the cheek, or zygomatic arch, is usually slender or deficient, the latter being the case in most of the species, and post-orbital processes of the frontals are found only in the Tupaiidae and Macroscelididae. The number of dorsal vertebrae varies from 13 in Tupaia to 19 in Centetes, of lumbar from 3 in Chrysochloris to 6 in Talpa and Sorex, and of caudal from the rudimentary vertebrae of Centetes to the 40 or more well-developed ones of Microgale.
The breast-bone, or sternum, is variable, but generally narrow, bilobate in front and divided into segments. The shoulder-girdle presents extreme adaptive modifications in the mole, in relation to the use of the fore-limbs in burrowing; but in the golden moles the fore-arm and fore-foot alone become specially modified. In Macroscelides the bones of the fore-arm are united at their lower ends, but in all other Insectivora the radius and ulna are distinct. The fore-foot has generally five digits; but in Rhynchocyon and in one species of Oryzorictes the first toe is absent, and in the moles it is extremely modified. The femur has, in most species, a prominent ridge below the greater trochanter presenting the characters of a third trochanter. In Tupaia, Centetes, Hemicentetes, Ericulus and Solenodon the tibia and fibula are distinct, but in most other genera united. The hind-foot consists usually of five digits (rarely four by reduction of the first), and in some, as in the leaping species (Macroscelides, Rhynchocyon), the tarsal bones are elongated. The form of the pelvis, and especially of the symphysis pubis, varies within certain limits, so that while in the Tupaiidae and Macroscelididae there is a long symphysis, in the Erinaceidae, Centetidae and Potamogalidae it is short, and in the Soricidae, Talpidae and Chrysochloridae there is none.
Owing to the similarity in the character of the food, the truly insectivorous species, forming more than nine-tenths of the order, present little variety in the structure of the digestive organs. The stomach is a simple, thin-walled sac; sometimes as in Centetes, with the pyloric and oesophageal openings close together; the intestinal canal has much the same calibre throughout, and varies from three (in the shrews) to twelve times (in the hedgehogs) the length of the head and body. In the arboreal Tupaia and the allied Macroscelididae, which probably feed on vegetable substances as well as insects most of the species possess a caecum. The liver is deeply divided into lobes, the right and left lateral being cut off by deep fissures; both the caudate and Spigelian lobes are generally well developed, and the gall-bladder, usually large and globular, is placed on the middle of the posterior surface of the right central lobe.
All the members of the order appear to be highly prolific, the number of young varying from two to eight in the hedgehog, and from twelve to twenty-one in the tenrec. The position of the milk-glands and the number of teats vary greatly. In Solenodon there is a single pair of post-inguinal teats, but in most species these organs range from the thorax to the abdomen, varying from two pairs in Gymnura to twelve in the tenrec. In the golden moles the thoracic and inguinal teats are lodged in deep cut-shaped depressions.
Scent-glands exist in many species. In most shrews they occur on the sides of the body at a short distance behind the axilla, and their exudation is probably protective, as few carnivorous animals will eat their dead bodies. In both species of Gymnura and in Potamogale large pouches are situated on each side of the rectum, and discharge their secretions by ducts, opening in the first-named genus in front of and in the latter within the margin of the vent. In the tenrec similarly situated glands discharge by pores opening at the bottom of deep pits.
The skin is thin, but in many species lined with well-developed muscles, which are probably more developed in hedgehogs than in any other mammals. In this family and in the tenrec most of the species are protected by spines implanted in the skin-muscle, or panniculus carnosus.
The Insectivora may be divided into two groups, according to the degree of development of the union between the two halves of the pelvis. The first group is characterized by the full development of this union, both pubis and ischium Tree-Shrews. entering into the symphysis. The tympanum remains as a ring within an auditory bulla; the orbit is either surrounded by bone, or separated from the hinder part of the skull by a post-orbital process of the frontal; the upper molars have broad 5-cusped crowns with a W-shaped pattern; and the intestine is generally furnished with a caecum. The first family of this group is the Tupaiidae, represented by the tree-shrews, or tupaias, of the Indo-Malay countries, characterized by the complete bony ring round the eye-socket, the freedom of the fibula from the tibia in the hind-limb, and the absence of any marked elongation of the tarsus. The dental formula is i. 2⁄3, c. 1⁄1, p. 3⁄3, m. 3⁄3, total 38. In appearance and habits tree-shrews are extremely like squirrels, although they differ, of course, in toto as regards their dentition. A large number of species are included as the typical genus Tupaia, which ranges from north-eastern India to the great Malay Islands. In these animals the tail has a fringe of long hairs on opposite sides throughout its length. In the pen-tailed, tree-shrew (Ptilocercus lowii), fig. 1, the only representative of its genus, and a native of Sumatra, Borneo and the Malay Peninsula, the fringes of long hair are confined to the terminal third of the tail. There are also differences in the skulls of the two genera. A third genus, Urogale, represented by U. cylindrura of the mountains of Mindanao, in the Philippines, and U. everetti, of Borneo, has been established for the round-tailed tupaias, in which the tail is uniformly short-haired, and the second upper incisor and the lower canines are unusually large, the third lower incisor being proportionately small, and also erect, while the second upper incisor resembles a canine. (See [Tree-Shrew].)