SUB ORDER SERPENTES (OPHIDIA)
These order animals have a narrow, cylindrical & elongated body, usually tapering towards the posterior end. Demarcations of head, trunk & body are not very obvious, even though they consist of them. The head consist of special scales & the rest of the bodies usually have scales forming different patterns. Absence of limbs is the most peculiar character shown by them.
Snakes also lack external ears. They are very sensitive to low frequency vibrations (200-500Hz). The waves go through the quadrate born & the columella to the inner ear. Snakes do not have movable eye lids which makes it easier to identify snakes from other reptiles which lack limbs. Most snakes don’t have colour vision. Long sightedness & stereoscopic vision is present in arboreal snakes. Terrestrial snakes usually show a short sightedness. Some sea snakes have cutaneous photoreception & become sensitive to light. Most crepuscular snakes have a vertical pupil, diurnal have round pupils.
All snakes are carnivores & predators. Snakes may immobilize or kill prey before swallowing it. They may take fresh kills. Active foraging & sit & wait foraging is also shown when catching prey. They eat eggs of other animals & may even show cannibalism & carrion feeding. Snakes mainly feed on rodents, reptiles, fishes, frogs & insects. Most inject their venom or venomous saliva to paralyze the prey. Some use the constrict method in immobilizing prey. They coil around the thoracic region preventing rib movement & suffocate the prey to death. Most don’t tear the prey in to peaces but swallow the whole thing & gain the ability to stay without eating any food for days.
Snake skull is highly kinetic. It enables them snakes to swallow prey several times their own diameter. They lack the jugal bone & the upper temporal bone. It helps them to open their jaws as any amount needed. Mandibles are only joined by a ligament, giving the ability to move freely. Curved teeth & muscles help them to take in a prey which is larger than their own size. Trachea starts from the anterior end of the mouth, so it can breathe still even when it has a prey in the mouth. Swallowing of large prey involves other modifications, such as, loss of pectoral griddles. Skin & scales are also designed to allow great expansion in body diameter.
Most snakes use chemical sense to detect prey. Jacobson organ (vomeronasal organ) helps in this matter. It is lined with olfactory epithelium & is highly innervated by nerves. The tongue is forked towards the tip & protruded through air picking up the chemical scent particles, & is withdrawn & delivers the chemicals to the Jacobson organ. Then the information is transmitted to the brain, in order to identify the scent.
Tubercles & apical pits work as thermo receptors, photoreceptors & mechanoreceptors. Loreal pits are thermo receptors & labial pits are photo receptors. Some snakes show scale polishing which also help them in sensing.
There are highly venomous, moderately venomous, mildly venomous, & non venomous snakes found in Sri Lanka. Teeth are present on the maxillae, palatines & pterygoids & the mandible. In poisonous snakes the maxillary teeth become fangs which leave large two circular punctures when bitten & have poison apparatus in the head. Non venomous snakes possess solid, elongated, sharp, pointed & usually re-curved teeth which leave many small pricks when bitten.
Snakes venoms are six types; haematotoxins (blood), neurotoxins (nerve system), nphrotoxins (kidney), cardiovascular toxins (heart), myotoxins (muscles), endocrine toxins (hormone system). Most snakes’ venoms are complex mixtures of various fractions that attack different organs in specific ways.
Some types of snakes show sexual dimorphism such as some males having a darker colour & having keels on their scales may be on the ischiadic region; some times keels grow in to spines. Also when the rostral appendage is present, it’s changed according to sex. Back side of cloaca, at the base of the tail is broad & the tail is long in males. Except for male snakes that do the combat dance, other males are shorter than the female.
Most females release a chemical known as pheromones to attract males in certain time for the mating season. Some show a courtship dance. Male insert its hemipenis in to the female cloaca when at the sexual act. Snakes rarely do show pathenogenicaly.
Their reproduction occurs in 2 ways, such as oviparous (eggs are laid by the female & the young uses the egg tooth to break the calcareous shell) & ovoviviparous (eggs hatch while in side the female & live young off spring comes out). Some snakes incubate their eggs & also some even protect them. Some hang around their mother for few days though they all have the ability to find food on their own.
When threatened they mostly escape by rapidly or may not move at all. Some hide, some hide the head. Balling, death feigning, mimicry, caudal autotomy, autohaemorrhagy are also shown in some snake types. Some tries to scare the predator may be by making sounds (rattle), also gapping. Some fight for their life & some have the ability of spitting venom. Snakes show aposematic colouration & scare the threat off.
Their mobility occurs in 4 ways, such as, horizontal undulatory progression, reticular progression, cocertina progression, & sidewinding. Most snakes can swim & they have a muscular flap to close the nostrils while in water. Oar like tails are used by sea snakes to swim. Prehensile tails are used by arboreal snakes to hang on trees. Some may even show the ability of gliding.
Snakes are known as the most threatened reptiles of all, mainly by humans. It should always be noted not all snakes are poisonous & even if they are poisonous they may not hurt you if you do not try to hurt them. Sri Lanka contains a very high diversity of them, so we should protect them.
There have been comprehensive publications on the snakes of Sri Lanka in the past. Yet and easy reference guide to key characters such as endemicity distribution patterns and classification based on venom, is no available. The following paper was published with the view of fulfilling this requirement.
Sri Lanka has 97 species of snakes of which, 46 species are endemic., 5 endemic genera and 5 endemic sub species. All members of the UROPELTIDAE and CYLINDROPHIDAE family being endemic. Out of the 10 members of family TYPHLOPIDAE represented in Sri Lanka, 8 species are
endemic. (Gongylophis conica brevis) Sand Boa is an endemic sub species to the family BOIDAE. Out of the 45 members in family COLUBRIDAE 15 endemic species, 3 endemic genera and 3 endemic sub species are found.
Regarding family ELAPIDAE , the Sri Lankan Krait (Bungarus ceylonicus ceylonicus) is an endemic species, while the Ceylon Krait (Bungarus ceylonicus karavala) and the Sri Lanka Coral Snake (Calliophis melanurus sinhaleyus) are endemic sub species. Families ACROCHORDIDAE and HYDROPHIDAE Sea Snakes have not been represented by endemic species in Sri Lanka. Out of 6 species there are 3 endemic species in family VIPERIDAE.
The distribution patterns of Land Snakes were identified by using data collected from personal field records and references. The distribution of the Snake Fauna was concentrated in the Dry Zone, mainly in the Monsoon Forest and Grass Land (Map 2-B). The distribution in the Wet Zone was the next highest, with the majority of species observed in the South Western Rain Forest and Grass Land. The Intermediate Zone had the least number of records. Members of the family COLUBRIDAE were the most extensively distributed group in all 3 Zones. The table provided can be updated by further observations.
The Check List on Snakes also includes the category to which the snakes belong based on their venom. Out of the 24 venomous species documented, 7 species are Highly Poisonous (Can be fatal to man), 4 are Poisonous (Serious symptoms, very rarely fatal), 13 are Mildly Poisonous (Of no consequence to man). All members of the family HYDROPHIDAE are Deadly Poisonous