National Geographic : 1946 Sep
The National Geographic Magazine prefer their softer, juicier tissues. When there is a midge pupa in the gall, the larva then eats that. Even after I had seen much of its browsing in gall tissue, I was loath to believe it would eat flesh. But finally I found the larva at the entrance hole it had eaten in a gall, with a half-eaten midge pupa inside the gall. The larva when grown pupates in its bur row. On emergence as an adult moth, it leaves its empty pupal skin sticking out of the hole. This empty shell may easily be distinguished from that of the gall midge by lack of a horn on the front of the head and by having wing sheaths for two pairs of wings. Seed-eating larvae of at least four other kinds of moths live within the Bidens heads. Adults of all four came out of the heads I collected and put in my glass-jar rearing cages. Two are much like the Phalonia in size and habits. Two are larger kinds, a single one of which may devour all the seeds in a head. Such ravage by many individuals would be a scourge to the whole Bidens community, but the many parasites of these larger ones seem to keep their numbers down. The full-grown larvae leave the heads and spin their slender cocoons in a corner of the rearing cage, amid a tangle of silken threads that looks like the work of spiders. Colors Camouflage "Loopers" Other caterpillars live not in the heads but on them. These feed on the corollas. Be cause they do most of their eating after the flowers have been fertilized, they do the plant little injury. Two of them intrigued me by the perfection of their protective resemblance to fading corollas. They live out in the light where color counts, and their colors protect them well. Both are caterpillars of the kind popularly known as "loopers." Who has not had himself measured, inch by inch, for a new suit by a green "measuring worm" that had come down from a treetop to his shoulder on a long swaying thread of silk so fine that the thread was almost invisible? These Bidens loopers are very inactive. The one shown on page 353 sat very quietly for this photograph. It is the larva of a very beautiful pea-green moth (Synchlora denti culata) and is camouflaged by its own efforts. It fastens bits of corollas to the tubercles on its own back. Under these its concealment is perfect. I had been handling the flower heads in one of my rearing jars for several days before I saw one larva that must have been repeatedly before my eyes. The other is marked protectively by the colors in its skin: a ground color of brown, the shade of withered corollas, with yellowish quadrangular spots along the sides, one on each body segment, lined up like the windows in a railway coach. Two groups of animals, thrips and mites, are the most constant residents of all in Bidens flower heads. They are so minute as to re quire technique that I could not use with the equipment I had available. They are small enough to enter bodily into the crevices be tween flowers, or to go down in the depths of the corolla tubes and live there, adults and young together. Individually they are so small that the damage to the plant is usually negli gible. Aphids, Mealy Bugs, and Their Train Aphids and mealy bugs, being sluggish in sects, are unable to gain access to the rich stores of food within the Bidens heads and must content themselves with sucking sap from the outside. With their slender tubular beaks they penetrate to the sap channels of the plant. Gregarious, they habitually cluster around the base of the Bidens head and up its sides, where new-formed tissues are softest and where sap flows most freely (Plate VII). They are enormously prolific. Young and old remain together and quickly form colonies. Quite unarmed for defense, they invite aggres sion. As is well known, both aphids and mealy bugs secrete honeydew. The colonies are at tended by ants which gather the honey and, in a weak measure, herd the colonies, driving away some of their enemies (Plate VI).* Among their predatory enemies, a conspicu ously colored ladybird beetle (Plate IV) was much in evidence. Both the adult and its larva could be found devouring the helpless aphids, cleaning up entire colonies in a re markably short time. Another predator in the colonies of both aphids and mealy bugs is the larva of the flower fly that has the roistering name of Bac cha (Baccha clavata). It is very inconspicu ous. Though quite common, I discovered it only by first finding the big red ants that were attending aphid colonies, and then searching the flower heads near the ants. The Baccha larva is headless and footless, with a body nearly long enough to encircle the flower stalk at the base of a head. It seems to stay in place there constantly (Plate VI). At the tapering and pliant front end of the body it has a pair of murderous mouth hooks. * See W. M. August, "Stalking Ants, Savage and Civilized," by Mann, NATIONAL GEOGRAPHIC MAGAZINE, 1934.