Sunday, November 29, 2015

Success! Twice Over!

Blue Flea Beetle on leaf.  Hard to photograph - too reflective!
This shorter stay at La Selva is coming to an end, but it has been a good four-month stay. The effort to propagate cuttings of Passiflora lobata resulted two months later in 6 small but healthy potted plants with about 10 leaves each.   I was able to put these in cages with several Blue flea beetles, and, after a few weeks, got some eggs and larvae.  This was crucial, since the Blue Flea Beetle, with scientific name Monomacra violacea, is one of the most common flea beetles at La Selva.  It is also the most tolerant and can be found feeding on nearly every species of Passifora except two.  In past field seasons I found the larvae of the other genera, but Monomacra eluded me.

Monomacra species are obviously related to the three species of yellow Parchicola.  Both genera have a similar trim, elongated shape and both have a flattened rectangular depression on the posterior end of the pronotum, the "shield" immediately behind the head.  In 2013-14 I found eggs and larvae of two of the Parchicola species, and I expected the Blue beetles to be similar.  What I found was indeed some amount of similarity, but also some differences.

Like Parchicola "yellow-legged" (there is no official species name in this case), M. violacea cylindrical eggs are laid near the base of the plant.  Unlike Parchicola, the eggs are attached at the end (see photo).  The eggs hatch after an unknown number of days, and the slender, tiny larva emerges.  The larva is quite mobile, looping tail to head like a tiny inchworm.  The six thoracic legs are sturdy, grabbing the substrate easily.  The anal clasper is also relatively large and is surrounded by a ring of bristles. Each segment of the thorax and abdomen has two rows of pigmented flaps, like deflated balloons.  This pattern also contrasts with Parchicola, which have only one row of flaps.  Also, the larva is white rather than yellowish.  Otherwise, the young larvae of the two genera are similar.

Newly hatched M. violacea larva.
Newly hatched larvae have a transparent, unpigmented head capsule, bearing two large bulbous antennae capable of retracting into tubes on the head.  The legs and anal ring are also initially transparent, but after one day they melanize and darken.  The head capsule in particular become shiny black. The larvae loop along the rootlets of the host plant, feeding occasionally by chewing on root hairs and root tissue.  Typical feeding damage includes small pits eaten out of the pithy part of the root.  The central fibrous core of the rootlet is not eaten.
Third instar larva, almost fully grown and ready to pupate.

Above I was describing the first instar M. violacea larvae.  I have not yet seen the second instars, but the third instars are, of course, much bigger and fatter in relation to their body length.  The biggest change is that the pigmented flaps are replaced by tiny transparent spheres in the larger larvae.  They look like droplets of liquid but they are in fact cuticular, dry not wet.  The root environment where the larvae live has many predators such as centipedes, roundworms and isopods, and I can easily imagine the spheres are filled with some chemical deterrent.  But this just a guess at this time.

I forgot to explain the title of the blog: Success Twice Over! "  The eggs and larvae described here are one success.  The other has been my effort to make sense of the parallel species diversity of the flea beetles and the Heliconius butterflies.  More on that in the next blog.
Larval feeding damage to rootlet of P. lobata

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