Sil's Species

The "Species" in the movie Species is an unnamed, aggressive alien species intent on spreading itself throughout the universe by infiltrating and annihilating the intelligent populations of other planets. It is capable of rapid growth and can bond its own DNA to that of other creatures through a number of methods. They are apparently inspired by the mythological succubus and incubus, who were demons who seduced their victims to have sexual intercourse with them.

Origins
In Species II a professor named Herman Cromwell, played by Peter Boyle, claims that the original species originated in the Large Magellanic Cloud (called the Magellanic Galaxy). The only other place in the universe where Carbon-based life had apparently been discovered. It is also stipulated that they were a "cancerous" race that visited Mars millions of years ago when its surface was similar to that of Earth, and annihilated all life on its surface before leaving a remnant of DNA in its soil, in case something from another planet came to visit so that their species could continue to infect other planets in a similar manner.

Mankind's first interaction with this particular species occurred in 1994 when the SETI (Search for Extra-Terrestrial Life) project received a pair of messages from outer space. The first message contained a more efficient use of methane, while the second contained a DNA sequence along with directions on how to mix it with Human DNA. As the first message was seen as friendly contact, scientists went ahead and performed the fusion. Three living organisms were created, two of which were put into frozen stasis in liquid nitrogen, while the third was allowed to grow under the codename "Sil".

Physical Appearance and Growth
The Species appear to be sexually dimorphic with males being much larger than females. Males are also quadrupedal with females having smaller bipedal (humanoid) forms and appear to be much more agile. Both genders of the Species have skeletal bodies with their heads being much smaller compared to their bodies, their mouth has two mandibles similar to that of the Predalien and also have hair like mandibles that are similar to the Predator species. There was a noticeable change in the appearance of the males in Species III and the Patrick creature in Species 2, this is only because the host of the alien DNA in Species 2 was infected with the alien DNA so the resulting creature was a pure form of the creature. Both genders have the ability to shape shift between Human and Alien forms and also exhibit a limited ability to morph their alien forms for the purpose of adaptation to given situations. Females are able to manipulate their "hair" as well as retractable spikes on their backs used in combat and their breast tentacles as well. Males are able to split their heads in two, thus forming a duel headed form and also exposing a long tentacle from between their "heads" which can be used to kill or perhaps impregnate females. However, it is unknown whether the Species is patriarchal or matriarchal within its own social circles as a population of sufficient size has never been observed in the movies.

Most of the members of the species seen are actually mutations from humans, but a pure form is seen in the first film. It takes on the form of a blobby, tentacled monster with a long mouth-like appendage that lashed out at whatever was nearby and grew so fast that it would double in size in a matter of seconds. After a little over two minutes of life, it was killed by immolation.

In the Film Novelization of Species, the "pure" creature was actually a hybrid between the alien DNA and the DNA of a bat. This was done so that the creature would know no better than to use its natural free form instead of shielding itself behind a camouflage.

The most prominent example of a mutation induced by the species was that of "Sil". The alien imposed its rapid growth on Sil, causing her to form into a visible fetus in a single day and grow into a young girl (with the gender chosen to make her more "Docile", something that got a laugh from Press) on the verge of puberty in a month. She exhibited great speed and strength, but also suffered from terrible nightmares brought on by her alien DNA. Sil later claims that she thinks the nightmares "Tell me who I am".

While on the run, Sil pupates by forming a ball of maroon-colored flesh similar in appearance to the pure alien form. She was horrified by this development and the process appears to be very painful. After a few hours, she emerged as a fully-grown (and consequently very attractive) adult.

While still somewhat shy and uneasy, Sil is able to quickly adapt to her surroundings, showing an incredible cunning as she blends into society. Pretty soon, her alien libido takes over and she attempts to mate while in a state of almost constant fertility (she seems to menstruate once). During this time, she demonstrates a strong animalistic survival instinct, regeneration abilities, and an ability to sense genetic flaws in potential mates. She also kills without remorse when she feels threatened.

Her human form is in truth merely a disguise and her alien form is a hideous mockery of the human form. Her form is chitinous and reptilian, somewhat reminiscent of the aliens from Alien, but more human in appearance. Her hair has been replaced with tentacles which are slicked back and her teeth are sharp. Her breasts, rather than storing fat or mammary glands instead store tentacles which come out of her nipples, she uses them as a weapon but they are also used in a mating ritual (As shown in the second film). This form seems capable of holding its breath underwater for an extended period.

Her son shows some different traits and after being born less that an hour after conception grows at an incredible rate with limbs growing disproportionately and his spine seeming to grow too fast for his skin to keep up.

Since the boy's alien form was quadrupedal (as opposed to bipedal, like Sil's form) and more 'brutish' in appearance than hers, it is assumed that this must be the common appearance of the males of the alien species.

Technology
In the movie's Novelisation it is mentioned that the signal that came to Earth was transmitted through several Black Holes which served to hide its point of origin.

It is unknown what level of technological achievement "The Species" are currently at, however their mentioned mastery of black holes is a testament to their achievements. In the cannon versions of both movies and their novelizations, the Species have no space travel capabilities, however in a regected Species II script, a space faring vehicle of some kind belonging to the original Alien race is mentioned. It does seem plausible (although not canon) race advanced enough to manipulate Black holes would also have achieved advanced space travel.

In the second Movie Novelization "Species II". We learn that the species had once established a colony on Mars and consumed all of the planet's natural resources before moving on. It is unknown how they got to Mars and there is no mention of space travel leaving it open to debate. However, a character speculates that the Species could be literally billions of years old. Their capabilities remain unknown but the human hybrids show an ability to learn and absorb information extremely fast and put that knowledge to practical use.