Jaculus (plural Jaculi) is a later spelling of Iaculus, which appeared largely in medieval European manuscripts.  Iaculus (plural Iaculi) originated in Ancient Egypt or possibly Mesopotamia, likely derived from the ancient symbol of the Uraeon or Uraeus (plural Uraei or Uraeuses). 
Uraeus is from the Greek meaning 'on its tail,' which signifies the rearing position of the serpent.  The comparative Egyptian root is iaret, the rearing cobra.  The connection between the Egyptian iaret and its possible derivative Iaculus is unclear.
From Guardians to Javelins
While external influences seeped into Egypt through both conflict and trade, foreign influences on Egyptian mythology and religion were limited.  New religions or new beliefs would coexist with local Egyptian traditions but rarely mixed together. In some locations, the Egyptian belief systems, along with their religious ornaments and traditions, became more conservative in the presence of new ideas. 
However, foreign influence did seep into Egyptian belief systems over many generations,  especially influences from later Greek and Roman sources, which had previously been influenced by Egyptian myth. This slow influence is the likely reason that Iaculus evolved from the Uraeus, sacred to the Egyptians.
Some accounts believe that Iaculus is less of an evolution and more of a degeneration of the Uraeus. 
The winged serpents of Egypt connect to Nehebkau, the serpent god of the underworld, and the symbolic winged serpent of the Goddess Mersokar.
Superior form of Attack
Jaculus is taken from the Latin Jaculum, meaning 'javelin,' specifically donned onto this dragon for its peculiar method of attack.  A Jaculus would lie in wait up in the trees, and when a suitable target passed below, it would launch itself onto the target's back and kill by sinking its fangs in the neck.  Other accounts say the Jaculus coils itself up and fires at the target like a spring. 
Iaculi Serpentes. Hic semper ventis pernitior atque sagittis tranfigit quoque iaculatus ab arbore sese.
[Translation: Arrow or Dart Snakes. Always winds up destruction and like arrows throws itself from trees.]
-- Solinus, Collectanea rerum memorabilium, third century AD 
Eorum quidam similiter atque jacula semetipsos jaculantur, ex eoque quod agunt numen trahunt; nam jaculi nomina ntur.
[Translation: Some of them (serpents) like darts (or arrows, javelins) shoot themselves and so potent is the attack that they have dart for a name.]
-- Aelian, De Natura Animalium, second century AD 
Iaculus serpens volans.
[Translation: Arrow flying serpent.]
-- Isidori, Hispalensis Episcopi Etymologiarum sive Originum, sixth century AD 
Lo! Upon branchless trunk a serpent, named By Libyans Jaculus, rose in coils to dart His venom from afar. Through Paullus' brain It rushed, nor stayed; for in the wound itself Was death. Then did they know how slowly flies, Flung from a sling, the stone; how gently speed Through air the shafts of Scythia.
-- Lucan, Pharsalia, first century AD 
As concerning serpents... that the javelin-shake hurls itself from the branches of trees, and at serpents are not only formidable to the feet but fly like a missile from a catapult; that when asps' necks swell, up there is no remedy for their sting except the immediate amputation of the parts stung...
-- Pliny the Elder, Natural History, first century AD 
- Rose [Dragons] 187
- Rose [Dragons] 193
- Müller [Egypt] 239
- Müller [Egypt] 240
- Howey 1
- Howey 34
- Egyptian Myths - <http://www.egyptianmyths.net/cobra.htm>
- Nigg [Dragons] 43
- Solinus, Collectanea rerum memorabilium <http://archive.org/details/collectanearerum00soliuoft>.
- Aelian, De Natura Animalium <http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Aelian/home.html>.
- Isidori Hispalensis Episcopi Etymologiarum sive Originum <http://penelope.uchicago.edu/Thayer/L/Roman/Texts/Isidore/12*.html>.
- Lucan, Pharsalia, <http://ancienthistory.about.com/library/bl/bl_text_lucan_ix.htm>.
- Pliny, Natural History, <http://www.masseiana.org/pliny.htm>.
For more information on footnotes and references, please see the bibliography.