An insect larva (_Cochlophora valvata_) from E. Africa is said to resemble a _Valvata_ or young _Cyclostoma_. In this case the spiral is indifferently dextral or sinistral, the ‘shell’ being formed of masticated vegetable matter, united together by threads spun by the larva. Certain larvae of the Phryganeidae (“Caddis-worms”) enclose themselves in houses which more or less resemble a spiral shell, and have in some cases actually been described as molluscan; such species, some of which belong to _Helicopsyche_, have been noticed in S. Europe, Ceylon, Further India, China, Tasmania, New Zealand, Tennessee, Mexico, Central America, Venezuela, Brazil, and Argentina, and all[151] possess a dextral ‘shell.’ In all these cases ‘mimicry’ is probably not so much to be thought of as the practical advantages which accrue to the animal in question from the spiral form, which gives it greater strength to resist external blows, and enables it to occupy, during a very defenceless portion of its existence, a very small amount of space. The larva of some species of the Syrphidae (_Diptera_) fixes itself on the under side of stones in the Tyrol, and closely resembles a small slug. The naturalist Von Spix, in 1825, described to the Bavarian Academy as a new genus of land Mollusca a somewhat similar larval form found in decaying wood on the banks of a German lake.[152] Simroth mentions[153] a curious case as occurring near Grimma. The caterpillars of certain Microlepidoptera occur on slabs of porphyry, associated with a species of _Clausilia_. Besides being of the same colour as the Clausiliae, the caterpillars have actually developed cross lines on the back, _i.e._ on the side turned away from the rock, in imitation of the suture of the mollusc. It has been suggested[154] that there is mimicry between _Aeolis papillosa_ (a common British nudibranch) and _Sagartia troglodytes_ (an Actinian), and also between another species of _Sagartia_ and _Aeolidiella Alderi_. The facts observed are not sufficient to warrant a decided opinion, but it seems more probable that the Actinian mimics the nudibranch than _vice versâ_, since _Aeolis_ is known to be unpalatable to fishes. [Illustration: FIG. 28.--=A=, _Strombus mauritianus_ Lam., which mimics _Conus_ in shape. =B=, _Conus janus_ Hwass, Mauritius.]