Img 1: Cat flea pupa without a cocoon (naked pupa).
photo by: Auguste Le Roux
Naked pupae are pupae which develop without a cocoon. The cocoon isn’t necessary for survival and successful metamorphosis. Naked pupae develop when larvae can’t find a vertical surface to attach silk threads to, or when newly formed cocoons are shaken, which forces larvae to abandon their structures.
What are Naked Pupae?
Most fleas pupate within a cocoon. However, some develop without cocoons and are called “naked pupae”.
Img 2: Cat flea pupae within cocoons.
photo by: Marcelo de Campos Pereira
Naked Pupae vs Cocooned Pupae
Development & Survival
The cocoon isn’t necessary for survival and development. Under optimal temperature and humidity, naked pupae survive and complete their metamorphosis just as well as cocooned pupae. 96.5% of naked pupae will successfully reach adulthood.
Quiescence & Emergence
Cocooned adults can enter into a quiescent state, where they’ll remain inactive for up to 5 months. Naked pupae can’t enter into this dormant-like state. Without cocoons, the adults emerge from their pupal casing soon after reaching maturity.
Pre-emerged adults will quickly wake up and emerge from their cocoons upon sensing a nearby host. The two triggers are heat and physical pressure. A cocoon helps prevent fleas from detecting non-host stimuli, thus it minimizes their chances of emerging without a host around. Naked pupae don’t have this stimuli protection. They’ll emerge readily, even in the absence of a host.
When vacuuming, naked pupae are easier to remove from carpets than those enclosed in a cocoon. This is because cocoon threads get spun into the carpet fibers, making them adhere tightly.
Protection from Predators
Pupae are the most hardy life stage, but how much of this resilience is due to the cocoon is unknown. However, the silk structure does protect pupae from potential predators, such as cannibalizing flea larvae and ants.
What Causes Naked Pupae?
Inhibiting Cocoon Formation
Flea larvae can’t spin cocoons without a vertical surface. If they can’t align themselves to a vertical surface, less than 3% spin a cocoon.
In laboratories, when flea larvae are raised in flat-bottom containers, the larvae can only form cocoons against the container’s walls. In round-bottom containers, the larvae fail to form cocoons. But, in an effort to try, they’ll deposit silk strands across the container’s bottom. When glass beads are added to the containers, the larvae are able to vertical orient themselves with the tactile stimulus, and thus there is an increase in successful cocoon formation. The larger the beads, the more successful the cocoon formation.
Fig 1 Percentage of larvae which complete cocoons (y-axis) in a round-bottom container with different sizes of beads (x-axis).
Forcing Cocoon Abandonment
Shaking & Agitation
Flea larvae being pupating 18 hours after completing their cocoon. If disturbed prior to this time, the larvae may abandon their cocoons. If cocoons less than 12 hours old are gently shifted, then 13.7% the larvae will emerge. If vigorously shaken, 26.7% will emerge. If shaken again, another 2.2% emerged abandoned their cocoons. All in all, 42.6% of the larvae left their cocoons after gentle sifting and agitation. Larvae will also emerge from cocoons when direct pressure is applied.
In carpeted home environments, it’s may be possible to force the development of some naked pupae. Beater-bar vacuum cleaners cause vigorous agitation to carpet fibers, and make be sufficient in causing flea larvae to abandon their cocoons and develop as naked pupae.
Spinning a Second Cocoon
After abandoning their cocoons, 49.1% of larvae will spin a second cocoon, 42.9% develop as naked pupae, and 8.0% die. Larvae forced to construct a second cocoon will develop into lighter pupae and adults. And they’ll emerge significantly sooner than those that only spun a single cocoon, most likely as a result of their depleted energy reserves. Similarly, undernourished larvae may produce weaker cocoon structures